Material for the StableHLO spec

[jtristan]

I wanted to have a discussion to share small things that may be of use to the StableHLO team (@GleasonK , @sdasgup3 ) when they refine the specification.

Following is a fleshed-out grammar for EnumConstant:

EnumConstant  ::= ComparisonDirection
                | ComparisonType
		| Precision
		| FftType
		| ChannelType
		| RngDistribution
		| RngAlgorithm
		| Transpose
ComparisonDirection  ::= EQ | NE | GE | GT | LE | LT
ComparisonType  ::= FLOAT | TOTALORDER | SIGNED | UNSIGNED
Precision  ::= DEFAULT | HIGHEST | HIGH
FftType  ::= FFT | IFFT | RFFT | IRFFT
ChannelType  ::= DEVICE_TO_DEVICE | HOST_TO_DEVICE
RngDistribution  ::= UNIFORM | NORMAL
RngAlgorithm  ::= DEFAULT | THREE_FRY | PHILOX
Transpose  ::= NO_TRANSPOSE | TRANSPOSE | ADJOINT