Add support for Tensorflow SparseTensors: core classes.

[hertschuh]

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.

[fchollet]

Thanks for the PR!

[codecov]

Codecov Report

Patch coverage: 82.35% and project coverage change: +0.01% 🎉

Comparison is base (49e5b06) 70.90% compared to head (e15efe2) 70.92%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #839      +/-   ##
==========================================
+ Coverage   70.90%   70.92%   +0.01%     
==========================================
  Files         344      344              
  Lines       33300    33325      +25     
  Branches     6409     6417       +8     
==========================================
+ Hits        23612    23635      +23     
+ Misses       8093     8092       -1     
- Partials     1595     1598       +3     

Files Changed	Coverage Δ
keras_core/backend/jax/core.py	89.13% <50.00%> (-0.93%)	⬇️
keras_core/backend/numpy/core.py	94.07% <50.00%> (-1.39%)	⬇️
keras_core/backend/torch/core.py	91.59% <50.00%> (-0.79%)	⬇️
keras_core/backend/common/keras_tensor.py	58.15% <100.00%> (+0.29%)	⬆️
keras_core/backend/jax/__init__.py	100.00% <100.00%> (ø)
keras_core/backend/numpy/__init__.py	100.00% <100.00%> (ø)
keras_core/backend/tensorflow/__init__.py	100.00% <100.00%> (ø)
keras_core/backend/tensorflow/core.py	96.18% <100.00%> (+0.24%)	⬆️
keras_core/backend/torch/__init__.py	100.00% <100.00%> (ø)
keras_core/layers/core/input_layer.py	90.90% <100.00%> (+0.66%)	⬆️

... and 1 file with indirect coverage changes

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[fchollet]

LGTM, thanks