[RFC] Add support for int2 in StableHLO (#2403)

To be shared to OpenXLA Discuss shortly.

---------

Co-authored-by: Sergei Lebedev <[email protected]>

rfcs/20240620-int2.md

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+# FP8 in StableHLO
+
+Status: In review<br/>
+Initial version: 06/20/2024<br/>
+Last updated: 06/20/2024<br/>
+
+## Introduction
+
+Sub-byte integer types are commonly used for model quantization. StableHLO
+currently supports signed and unsigned 4-bit integer types. Here we propose to
+extend the specification to cover 2-bit integer types as well.
+
+## Proposal
+
+Change the Types section of the spec to include si2 (signed 2-bit integer) and
+ui2 (unsigned 2-bit integer):
+
+```text
+SignedIntegerType ::= 'si2' | 'si4' | 'si8' | 'si16' | 'si32' | 'si64'
+UnsignedIntegerType ::= 'ui2' | 'ui4' | 'ui8' | 'ui16' | 'ui32' | 'ui64'
+```
+
+Add 2 to the list of supported bit width for integer types
+
+```text
+Integer types can be either signed (si) or unsigned (ui) and have one of the
+supported bit widths (2, 4, 8, 16, 32 or 64).
+```