PR #21380: Add F4E2M1FN and F8E8M0FNU types

[copybara-service]

PR #21380: Add F4E2M1FN and F8E8M0FNU types

Imported from GitHub PR #21380

Previous PR #19096 was rolled back, re-trying.

This PR adds F4E2M1FN primitive type (4-bit float with 2 bits exponent and 1 bit mantissa), F8E8M0FNU primitive type (8-bit float with 8 bits exponent, no mantissa and no sign) and enables loads/stores in the same way S4/U4 type is implemented.

This will enable using microscaling (MX) formats (RFC), such as MXFP4.

F4E2M1FN
- Exponent bias: 1
- Maximum stored exponent value: 3 (binary 11)
- Maximum unbiased exponent value: 3 - 1 = 2
- Minimum stored exponent value: 1 (binary 01)
- Minimum unbiased exponent value: 1 − 1 = 0
- Has Positive and Negative zero
- Doesn't have infinity
- Doesn't have NaNs

Additional details:
- Zeros (+/-): S.00.0
- Max normal number: S.11.1 = ±2^(2) x (1 + 0.5) = ±6.0
- Min normal number: S.01.0 = ±2^(0) = ±1.0
- Min subnormal number: S.00.1 = ±2^(0) x 0.5 = ±0.5

F8E8M0FNU
- Exponent bias: 127
- Maximum stored exponent value: 254 (binary 1111'1110)
- Maximum unbiased exponent value: 254 - 127 = 127
- Minimum stored exponent value: 0 (binary 0000'0000)
- Minimum unbiased exponent value: 0 − 127 = -127
- Doesn't have zero
- Doesn't have infinity
- NaN is encoded as binary 1111'1111

Additional details:
- Zeros cannot be represented
- Negative values cannot be represented
- Mantissa is always 1

Related PRs:

• Add MX floating point types (f4E2M1FN, f6E2M3FN, f6E3M2FN, f8E8M0FNU) stablehlo#2582
• Add sub-byte data types: float4_e2m1fn, float6_e2m3fn, float6_e3m2fn jax-ml/ml_dtypes#181
• [APFloat] Add APFloat support for FP4 data type llvm/llvm-project#95392
• [MLIR] Add f4E2M1FN type llvm/llvm-project#108877
• Add float8_e8m0_fnu (E8M0) OCP MX scale format jax-ml/ml_dtypes#166
• [APFloat] Add APFloat support for E8M0 type llvm/llvm-project#107127
• [MLIR] Add f8E8M0FNU type llvm/llvm-project#111028
  Copybara import of the project:

--
d7e00c4 by Sergey Kozub skozub@nvidia.com:

Add F4E2M1FN and F8E8M0FNU types

Merging this change closes #21380

FUTURE_COPYBARA_INTEGRATE_REVIEW=#21380 from openxla:skozub/e2m1_e8m0 d7e00c4