Introduce QuantizedType (#1352)

StableHLO dialect currently supports quantization via:
  1) Supporting `quant.uniform` element types.
  2) Having dedicated ops like `uniform_quantize` / `uniform_dequantize`.
  3) Allowing regular ops like `add` / `convolution` to take quantized
tensors.

This support was inherited from MHLO when StableHLO was bootstrapped,
and MHLO support was motivated by mobile use cases and inherited from
TFLite.

As pointed out in #1149, StableHLO specification doesn't support
quantization at the moment, and this is an important gap that we would 
like to fix before StableHLO v1.0 (see #588).

To continue the discussion started in #1149 and to make progress towards
v1.0, this pull request:
  A) Adds QuantizedType to the StableHLO specification, modelled after
[TFLite quantization
spec](https://www.tensorflow.org/lite/performance/quantization_spec).
  B) To start a conversation about the applications of QuantizedType and
the semantics of quantized ops, proposes semantics for quantized `add`.

TFLite quantization spec doesn't cover everything. It specs constraints
on types (which we captured accordingly in this pull request), but it
doesn't go into describing semantics of quantized ops.

As a result, the proposed semantics for quantized `add` is intentionally
naive, as compared with the much more involved implementations in the
TensorFlow repository, e.g.:
  *
[tfl.add](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/lite/kernels/add.cc).
  *
[tf.UniformQuantizedAdd](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/core/kernels/uniform_quant_ops/uniform_quantized_add_op.cc).

upd: After community discussion, we removed the spec for quantized
`add` leaving that for future work, since further alignment is required.

---------

Co-authored-by: Eugene Burmako <burmako@google.com>

docs/spec.md

@@ -81,7 +81,7 @@ completely numeric to simplify generation of StableHLO programs.
 
 ```ebnf
 Type         ::= ValueType | NonValueType
-ValueType    ::= TensorType | TokenType | TupleType
+ValueType    ::= TensorType | QuantizedTensorType | TokenType | TupleType
 NonValueType ::= ElementType | FunctionType | StringType
 ```
 
@@ -116,6 +116,69 @@ types, for example, to include layouts
 ([#629](https://github.com/openxla/stablehlo/issues/629)) and sparsity
 ([#1078](https://github.com/openxla/stablehlo/issues/1078)).
 
+```ebnf
+QuantizedTensorType ::= 'tensor' '<' TensorShape QuantizedElementType '>'
+QuantizedElementType ::= '!quant.uniform' '<'
+                  QuantizationStorageType
+                  ['<' QuantizationStorageMin ':' QuantizationStorageMax '>']
+                  ':' QuantizationExpressedType
+                  [':' QuantizationDimension]
+                  ',' QuantizationParameters '>'
+QuantizationStorageType ::= IntegerType
+QuantizationStorageMin ::= IntegerConstant
+QuantizationStorageMax ::= IntegerConstant
+QuantizationExpressedType ::= FloatType
+QuantizationDimension ::= IntegerConstant
+QuantizationParameters ::= QuantizationParameter
+                         | '{' QuantizationParameter {',' QuantizationParameter} '}'
+QuantizationParameter ::= QuantizationScale ':' QuantizationZeroPoint
+QuantizationScale ::= FloatConstant
+QuantizationZeroPoint ::=  IntegerConstant
+```
+
+**Quantized element types** represent integer values of a **storage type** in
+the range from `storage_min` to `storage_max` (inclusive) that correspond to
+floating-point values of an **expressed type**. For a given integer value `i`,
+the corresponding floating-point value `f` can be computed as
+`f = (i - zero_point) * scale`, where `scale` and `zero_point` are called
+**quantization parameters**. The `storage_min` and `storage_max` are optional
+in the grammar, but have default values of `min_value(storage_type)` and
+`max_value(storage_type)` respectively. Quantized element types have the
+following constraints:
+
+* (C1) `num_bits(storage_type) < num_bits(expressed_type)`.
+* (C2) `type(storage_min) = storage_type`.
+* (C3) `type(storage_max) = storage_type`.
+* (C4) `min_value(storage_type) <= storage_min < storage_max <= max_value(storage_type)`.
+* (C5) For all `i`, `type(scales[i]) = expressed_type`.
+* (C6) For all `i`, `scales[i] > 0`.
+* (C7) For all `i`, `is_finite(scales[i])`.
+* (C8) For all `i`, `storage_min <= zero_points[i] <= storage_max`.
+* (C9) For all `i`, `type(zero_points[i]) = storage_type`.
+* (C10) `size(scales) = size(zero_points)`.
+* (C11) If `quantization_dimension` is empty, then `size(scales) = 1`.
+* (C12) If `quantization_dimension` is not empty, then
+  `0 <= quantization_dimension`.
+
+**Quantized tensor types** represent tensors with quantized elements. These
+tensors are exactly the same as regular tensors, except that their elements
+have quantized element types, instead of regular element types.
+
+In quantized tensors, quantization can be **per-tensor**, meaning, having
+one `scale` and `zero_point` for the entire tensor or can be **per-axis**,
+meaning, having multiple `scales` and `zero_points`, one pair per slice of
+a particular dimension `quantized_dimension`. More formally, in a tensor `t` of
+with per-axis quantization, there are `dim(t, quantized_dimension)` slices
+of the `quantized_dimension`: `t[:, ..., 0, ..., :], t[:, ..., 1, ..., :]`, etc.
+All elements in the `i`th slice use `scales[i]` and `zero_points[i]` as their
+quantization parameters. Quantized tensor types have the following constraints:
+
+* For per-tensor quantization:
+  * No additional constraints.
+* For per-axis quantization:
+  * (C12) `quantization_dimension < size(shape)`.
+  * (C13) `size(scales) = shape[quantization_dimension]`.
+
 ```ebnf
 TokenType ::= 'token'
 ```
@@ -173,10 +236,6 @@ values of type `tensor<T>`).
   and an **imaginary part** of the same **element type**. Supported complex
   types are `complex<f32>` (both parts are of type `f32`) and `complex<f64>`
   (both parts are of type `f64`).
-* In the future, we are also planning to introduce **quantized types** that
-  represent integer values obtained via uniform quantization of floating-point
-  values using given scales and zero points
-  ([#588](https://github.com/openxla/stablehlo/issues/588)).
 
 ```ebnf
 FunctionType ::= '(' [ValueType {',' ValueType}] ')' '->' '(' [ValueType {',' ValueType}] ')'