Skeleton of the StableHLO spec (#23)

Add a skeleton of the StableHLO spec

Here is the skeleton of the StableHLO specification: Presented as a draft to signify that this is a preliminary version of the specification and will follow additions in future w.r.t different ops.

docs/spec_draft.md

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+# StableHLO Specification Draft
+
+## Types
+
+Following are the supported element types in StableHLO:
+
+  * **Integer types**
+
+    * Signed integer with two’s complement representation. Referred to in the
+    document as `si<N>`, where the bit-width N ∊ {4, 8, 16, 32, 64}
+    * Unsigned integer referred to in the document as `ui<N>`, where the
+    bit-width N ∊ {4, 8, 16, 32, 64}
+  * **Boolean types** referred to in the document as `pred`
+  * **Floating-point types**
+    * Single precision `f32`, double precision `f64` and half precision `f16`
+    floating-points complying with [IEEE 754
+    format](https://ieeexplore.ieee.org/document/8766229).
+    * Bfloat16 `bf16` floating-point complying with [Brain Floating-Point Format](https://cloud.google.com/blog/products/ai-machine-learning/bfloat16-the-secret-to-high-performance-on-cloud-tpus).
+    Provides the same number of exponent bits as `f32`, so that it matches its
+    dynamic range, but with greatly reduced precision.
+ * **Complex types** represents a pair of floating-point types. Supported ones
+ are `c64` (represents paired `f32`) and `c128` (represents paired `f64`).
+
+StableHLO supports a Tensor type `tensor`, to model the type of a n-dimensional
+array, represented in the opset as `tensor<SxE>` such that
+
+  * Shape `S` is a list of number of elements in each of the
+  dimensions and represented, in increasing order of the corresponding dimension
+  number, as an array of values of type `ui64`. A 0-value represents missing or
+  empty data in the corresponding dimension.
+  * Element type `E` is any one of the supported element types mentioned above.
+
+## Programs
+
+StableHLO programs consist of functions. Each function has arguments and results
+of supported types and a list of ops in static single-assignment (SSA) form
+which is terminated by a return op which produces the results of the function.
+StableHLO ops take operands and produce results.
+
+```
+ml_program.func @example_func(%arg: tensor<4x16xf32>) -> tensor<4x16xf32> {
+ %1 = stablehlo.floor %arg : tensor<4x16xf32>
+ %2 = stablehlo.ceil %arg : tensor<4x16xf32>
+ %3 = stablehlo.add %1, %2 : tensor<4x16xf32>
+ ml_program.return %3 : tensor<4x16xf32>
+}
+```
+
+A program is executed by passing argument values to a given function and
+computing result values. Result values of a function are computed by evaluating
+the graph of ops rooted in the corresponding return op. The evaluation order is
+implementation-defined, as long as ops are evaluated before their uses. Possible
+execution orders of the above example program are `%1` → `%2` → `%3` → `return`
+or `%2` → `%1` → `%3` → `return`.
+
+## Constants
+
+The section describes the constants supported in StableHLO along with their
+syntax.
+
+  * **Integer Constants** Standard integers, e.g. `123`, are constants of the
+  integer type (signed or unsigned). Negative numbers can be used with signed
+  integer types.
+  * **Boolean Constants** `true` and `false` are both valid constants of the
+  `pred` type.
+  * **Floating-point Constants** Floating-point constants use standard decimal
+  notation, e.g. `123.421`, exponential notation, e.g. `1.23421e+2`, or a more
+  precise hexadecimal notation, e.g. `0x42f6d78d`.
+  * **Complex Constants** Complex constants are represented as a pair of real
+  and imaginary values of `f32` or `f64` types, e.g. `(12.34, 56,78)`.
+
+## Structure of an Op’s Specification
+
+The specification of an op comprises of the following components (in the order
+    described below)
+
+  * **Syntax** Operation mnemonic and its signature.
+  * **Semantics** Semantics of the operation.
+  * **Operands** Meaning of operand(s) and their type(s).
+  * **Results** Meaning of the result(s) and the type(s).
+  * **Constraints** Type constraints on the operand(s), result(s).
+  * **Examples** Examples demonstrating the working of the op.
+
+## stablehlo.add
+
+`stablehlo.add(lhs, rhs) -> result`
+
+### Semantics
+
+Performs element-wise addition of two tensors `lhs` and `rhs` and produces a
+`result` tensor. For integer element types, if the element-wise sum has an
+unsigned/signed overflow/underflow, the result is implementation defined and one
+of the followings:
+
+  * mathematical result modulo $2^n$, where n is the bit width of the result.
+  * saturation to $2^{n-1} - 1$ (or $-2^{n-1}$) for signed overflow (or signed
+      underflow) and saturation to $2^n - 1$ (or $0$) for unsigned overflow (or
+        unsigned underflow).
+
+For floating-point element types, corner cases are defined by the IEEE-754
+specification.
+
+### Arguments
+
+| Operand Name(s) | Type |
+|-|-|
+| `lhs` | `tensor` of Integer, Floating-point and Complex element types |
+| `rhs` | `tensor` of Integer, Floating-point and Complex element types |
+
+### Results
+
+| Result Name | Type |
+|-|-|
+| `result` | `tensor` of Integer, Floating-point, and Complex element types |
+
+### Constraints
+
+  * `lhs`, `rhs` have the same type.
+  * Supported shapes: all static shapes.
+  * `result` must have the same type as that of `lhs` (or `rhs`).
+
+### Examples
+
+```c
+// %x: [[1, 2], [3, 4]]
+// %y: [[5, 6], [7, 8]]
+%z = stablehlo.add %x, %y : tensor<2x2xf32>
+// %z: [[6, 8], [10, 12]]
+```