Add support for Tensorflow SparseTensors: core classes.

This adds:
- Support for specifying  sparse in `KerasTensor` and `Input`.
- A boolean flag `backend.SUPPORTS_SPARSE_TENSORS`.
- Support for `tf.SparseTensor` is Tensorflow core ops.

keras_core/backend/common/keras_tensor.py

@@ -28,11 +28,19 @@ class KerasTensor:
     dtype is called "static shape inference".
     """
 
-    def __init__(self, shape, dtype="float32", record_history=True, name=None):
+    def __init__(
+        self,
+        shape,
+        dtype="float32",
+        sparse=False,
+        record_history=True,
+        name=None,
+    ):
         from keras_core import backend
 
         self.shape = backend.standardize_shape(shape)
         self.dtype = backend.standardize_dtype(dtype)
+        self.sparse = sparse
         self.name = name or auto_name(self.__class__.__name__)
         self.record_history = record_history
 
@@ -106,7 +114,7 @@ def __tf_tensor__(self, dtype=None, name=None):
     def __repr__(self):
         return (
             f"<KerasTensor shape={self.shape}, dtype={self.dtype}, "
-            f"name={self.name}>"
+            f"sparse={self.sparse}, name={self.name}>"
         )
 
     def __iter__(self):

keras_core/backend/common/keras_tensor_test.py

@@ -9,9 +9,10 @@
 
 class KerasTensorTest(testing.TestCase):
     def test_attributes(self):
-        x = keras_tensor.KerasTensor(shape=(3,), dtype="float32")
+        x = keras_tensor.KerasTensor(shape=(3,), dtype="float32", sparse=True)
         self.assertEqual(x.dtype, "float32")
         self.assertEqual(x.shape, (3,))
+        self.assertEqual(x.sparse, True)
 
     def test_numpy_methods(self):
         x = keras_tensor.KerasTensor(shape=(3, 2), dtype="float32")

keras_core/backend/jax/__init__.py

@@ -5,6 +5,7 @@
 from keras_core.backend.jax import nn
 from keras_core.backend.jax import numpy
 from keras_core.backend.jax import random
+from keras_core.backend.jax.core import SUPPORTS_SPARSE_TENSORS
 from keras_core.backend.jax.core import Variable
 from keras_core.backend.jax.core import cast
 from keras_core.backend.jax.core import compute_output_spec

keras_core/backend/jax/core.py

@@ -14,6 +14,8 @@
 from keras_core.backend.jax import distribution_lib
 from keras_core.utils.nest import pack_sequence_as
 
+SUPPORTS_SPARSE_TENSORS = False
+
 
 class Variable(KerasVariable):
     def _initialize(self, value):
@@ -44,7 +46,9 @@ def __jax_array__(self):
         return self.value
 
 
-def convert_to_tensor(x, dtype=None):
+def convert_to_tensor(x, dtype=None, sparse=False):
+    if sparse:
+        raise ValueError("`sparse=True` is not supported with jax backend")
     if dtype is not None:
         dtype = standardize_dtype(dtype)
     if isinstance(x, Variable):

keras_core/backend/numpy/__init__.py

@@ -4,6 +4,7 @@
 from keras_core.backend.numpy import nn
 from keras_core.backend.numpy import numpy
 from keras_core.backend.numpy import random
+from keras_core.backend.numpy.core import SUPPORTS_SPARSE_TENSORS
 from keras_core.backend.numpy.core import Variable
 from keras_core.backend.numpy.core import cast
 from keras_core.backend.numpy.core import compute_output_spec

keras_core/backend/numpy/core.py

@@ -7,6 +7,8 @@
 from keras_core.backend.common.stateless_scope import StatelessScope
 from keras_core.utils.nest import pack_sequence_as
 
+SUPPORTS_SPARSE_TENSORS = False
+
 
 class Variable(KerasVariable):
     def _initialize(self, value):
@@ -23,7 +25,9 @@ def __array__(self):
         return self.value
 
 
-def convert_to_tensor(x, dtype=None):
+def convert_to_tensor(x, dtype=None, sparse=False):
+    if sparse:
+        raise ValueError("`sparse=True` is not supported with numpy backend")
     if dtype is not None:
         dtype = standardize_dtype(dtype)
     if isinstance(x, Variable):

keras_core/backend/tensorflow/__init__.py

@@ -5,6 +5,7 @@
 from keras_core.backend.tensorflow import numpy
 from keras_core.backend.tensorflow import random
 from keras_core.backend.tensorflow import tensorboard
+from keras_core.backend.tensorflow.core import SUPPORTS_SPARSE_TENSORS
 from keras_core.backend.tensorflow.core import Variable
 from keras_core.backend.tensorflow.core import cast
 from keras_core.backend.tensorflow.core import compute_output_spec

keras_core/backend/tensorflow/core.py

@@ -12,6 +12,8 @@
 from keras_core.backend.common.stateless_scope import StatelessScope
 from keras_core.utils.naming import auto_name
 
+SUPPORTS_SPARSE_TENSORS = True
+
 
 class Variable(
     KerasVariable,
@@ -70,15 +72,28 @@ def _write_object_proto(self, proto, options):
         return self.value._write_object_proto(proto, options)
 
 
-def convert_to_tensor(x, dtype=None):
+def convert_to_tensor(x, dtype=None, sparse=True):
+    """Convert to a TensorFlow tensor.
+
+    `sparse=True` means that `tf.SparseTensor`s are returned as-is, which is the
+    default with the TensorFlow backend. An explicit `sparse=False` densifies
+    `tf.SparseTensor`s.
+    """
+    if isinstance(x, tf.SparseTensor) and not sparse:
+        x = tf.sparse.to_dense(x)
     if dtype is not None:
         dtype = standardize_dtype(dtype)
-        if tf.is_tensor(x):
-            return tf.cast(x, dtype=dtype)
-    return tf.convert_to_tensor(x, dtype=dtype)
+    if not tf.is_tensor(x):
+        return tf.convert_to_tensor(x, dtype=dtype)
+    elif dtype is not None:
+        return tf.cast(x, dtype=dtype)
+    else:
+        return x
 
 
 def convert_to_numpy(x):
+    if isinstance(x, tf.SparseTensor):
+        x = tf.sparse.to_dense(x)
     return np.array(x)
 
 
@@ -95,7 +110,8 @@ def shape(x):
     tensor values when the shape is unknown (this is tf specific, as dynamic
     shapes do not apply in other backends).
     """
-    x = tf.convert_to_tensor(x)
+    if not tf.is_tensor(x):
+        x = tf.convert_to_tensor(x)
     dynamic = tf.shape(x)
     if x.shape == tf.TensorShape(None):
         raise ValueError(

keras_core/backend/torch/__init__.py

@@ -20,6 +20,7 @@
 from keras_core.backend.torch import nn
 from keras_core.backend.torch import numpy
 from keras_core.backend.torch import random
+from keras_core.backend.torch.core import SUPPORTS_SPARSE_TENSORS
 from keras_core.backend.torch.core import Variable
 from keras_core.backend.torch.core import cast
 from keras_core.backend.torch.core import compute_output_spec

keras_core/backend/torch/core.py

@@ -12,6 +12,8 @@
 from keras_core.backend.common.stateless_scope import StatelessScope
 from keras_core.utils.nest import pack_sequence_as
 
+SUPPORTS_SPARSE_TENSORS = False
+
 # Some operators such as 'aten::_foreach_mul_.Scalar'
 # are not currently implemented for the MPS device.
 # check https://github.com/pytorch/pytorch/issues/77764.
@@ -118,7 +120,9 @@ def __eq__(self, other):
             return False
 
 
-def convert_to_tensor(x, dtype=None):
+def convert_to_tensor(x, dtype=None, sparse=False):
+    if sparse:
+        raise ValueError("`sparse=True` is not supported with torch backend")
     if is_tensor(x):
         device = get_device()
         if x.device != device:

keras_core/layers/core/input_layer.py

@@ -13,12 +13,13 @@ def __init__(
         shape=None,
         batch_size=None,
         dtype=None,
+        sparse=None,
         batch_shape=None,
         input_tensor=None,
         name=None,
         **kwargs,
     ):
-        # TODO: support for sparse, ragged.
+        # TODO: support for ragged.
         super().__init__(name=name)
         if "input_shape" in kwargs:
             warnings.warn(
@@ -45,6 +46,13 @@ def __init__(
         self.batch_shape = tuple(batch_shape)
         self._dtype = backend.standardize_dtype(dtype)
 
+        self.sparse = bool(sparse)
+        if self.sparse and not backend.SUPPORTS_SPARSE_TENSORS:
+            raise ValueError(
+                "`sparse=True` is not supported with backend: "
+                f"{backend.backend()}"
+            )
+
         if input_tensor is not None:
             if not isinstance(input_tensor, backend.KerasTensor):
                 raise ValueError(
@@ -54,7 +62,7 @@ def __init__(
                 )
         else:
             input_tensor = backend.KerasTensor(
-                shape=batch_shape, dtype=dtype, name=name
+                shape=batch_shape, dtype=dtype, sparse=sparse, name=name
             )
         self._input_tensor = input_tensor
         Node(operation=self, call_args=(), call_kwargs={}, outputs=input_tensor)
@@ -71,6 +79,7 @@ def get_config(self):
         return {
             "batch_shape": self.batch_shape,
             "dtype": self.dtype,
+            "sparse": self.sparse,
             "name": self.name,
         }
 
@@ -80,6 +89,7 @@ def Input(
     shape=None,
     batch_size=None,
     dtype=None,
+    sparse=None,
     batch_shape=None,
     name=None,
     tensor=None,
@@ -104,6 +114,11 @@ def Input(
         batch_size: Optional static batch size (integer).
         dtype: The data type expected by the input, as a string
             (e.g. `"float32"`, `"int32"`...)
+        sparse: A boolean specifying whether the expected input will be sparse
+            tensors. Note that, if `sparse` is `False`, sparse tensors can still
+            be passed into the input - they will be densified with a default
+            value of 0. This feature is only supported with the Tensorflow
+            backend. Defaults to `False`.
         name: Optional name string for the layer.
             Should be unique in a model (do not reuse the same name twice).
             It will be autogenerated if it isn't provided.
@@ -127,6 +142,7 @@ def Input(
         shape=shape,
         batch_size=batch_size,
         dtype=dtype,
+        sparse=sparse,
         batch_shape=batch_shape,
         name=name,
         input_tensor=tensor,

keras_core/layers/core/input_layer_test.py

@@ -1,29 +1,52 @@
 import numpy as np
+import pytest
+from absl.testing import parameterized
 
+from keras_core import backend
 from keras_core import testing
 from keras_core.backend import KerasTensor
 from keras_core.layers import InputLayer
 
 
-class InputLayerTest(testing.TestCase):
+class InputLayerTest(testing.TestCase, parameterized.TestCase):
     # Testing happy path for layer without input tensor
-    def test_input_basic(self):
+    @parameterized.named_parameters(
+        [
+            {"testcase_name": "dense", "sparse": False},
+            {"testcase_name": "sparse", "sparse": True},
+        ]
+    )
+    def test_input_basic(self, sparse):
         input_shape = (2, 3)
         batch_size = 4
         dtype = "float32"
         ndim = len(tuple((batch_size,) + input_shape))
 
-        values = InputLayer(
-            shape=input_shape, batch_size=batch_size, dtype=dtype
-        )
+        init_kwargs = {
+            "shape": input_shape,
+            "batch_size": batch_size,
+            "dtype": dtype,
+            "sparse": sparse,
+        }
+
+        if sparse and not backend.SUPPORTS_SPARSE_TENSORS:
+            with self.assertRaisesRegex(
+                ValueError, "`sparse=True` is not supported"
+            ):
+                InputLayer(**init_kwargs)
+            return
+
+        values = InputLayer(**init_kwargs)
 
         self.assertEqual(values.dtype, dtype)
         self.assertEqual(values.batch_shape[0], batch_size)
         self.assertEqual(values.batch_shape[1:], input_shape)
+        self.assertEqual(values.sparse, sparse)
         self.assertEqual(values.trainable, True)
         self.assertIsInstance(values.output, KerasTensor)
         self.assertEqual(values.output.ndim, ndim)
         self.assertEqual(values.output.dtype, dtype)
+        self.assertEqual(values.output.sparse, sparse)
 
     # Testing shape is not None and batch_shape is not None condition
     def test_input_error1(self):

keras_core/ops/core_test.py

@@ -1,6 +1,7 @@
 import numpy as np
 import pytest
 
+from keras_core import backend
 from keras_core import layers
 from keras_core import losses
 from keras_core import models
@@ -270,6 +271,18 @@ def test_shape(self):
         x = KerasTensor((None, 3, None, 1))
         self.assertAllEqual(core.shape(x), (None, 3, None, 1))
 
+    @pytest.mark.skipif(
+        not backend.SUPPORTS_SPARSE_TENSORS,
+        reason="Backend does not support sparse tensors.",
+    )
+    def test_shape_sparse(self):
+        import tensorflow as tf
+
+        x = tf.SparseTensor(
+            indices=[[0, 0], [1, 2]], values=[1.0, 2.0], dense_shape=(2, 3)
+        )
+        self.assertAllEqual(core.shape(x), (2, 3))
+
     def test_convert_to_tensor(self):
         x = np.ones((2,))
         x = ops.convert_to_tensor(x)
@@ -284,6 +297,32 @@ def test_convert_to_tensor(self):
         with self.assertRaises(ValueError):
             ops.convert_to_numpy(KerasTensor((2,)))
 
+    @pytest.mark.skipif(
+        not backend.SUPPORTS_SPARSE_TENSORS,
+        reason="Backend does not support sparse tensors.",
+    )
+    def test_convert_to_tensor_sparse(self):
+        import tensorflow as tf
+
+        x = tf.SparseTensor(
+            indices=[[0, 0], [1, 2]], values=[1.0, 2.0], dense_shape=(2, 3)
+        )
+
+        x_default = ops.convert_to_tensor(x)
+        self.assertIsInstance(x_default, tf.SparseTensor)
+        self.assertAllClose(x, x_default)
+        # Note that ops.convert_to_tensor does not expose the 'sparse' arg
+        x_sparse = backend.convert_to_tensor(x, sparse=True)
+        self.assertIsInstance(x_sparse, tf.SparseTensor)
+        self.assertAllClose(x, x_sparse)
+        x_dense = backend.convert_to_tensor(x, sparse=False)
+        self.assertNotIsInstance(x_dense, tf.SparseTensor)
+        self.assertAllClose(x, x_dense)
+
+        x_numpy = ops.convert_to_numpy(x)
+        self.assertIsInstance(x_numpy, np.ndarray)
+        self.assertAllClose(x_numpy, x_dense)
+
     def test_cond(self):
         t = ops.cond(True, lambda: 0, lambda: 1)
         self.assertEqual(t, 0)