Imported from GitHub PR openxla/xla#21380

Previous PR openxla/xla#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](openxla/xla#18085)), such as MXFP4.

```c
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:
- openxla/stablehlo#2582
- jax-ml/ml_dtypes#181
- llvm/llvm-project#95392
- llvm/llvm-project#108877
- jax-ml/ml_dtypes#166
- llvm/llvm-project#107127
- llvm/llvm-project#111028
Copybara import of the project:

--
d7e00c49a4b4f26c06266d6bb941275e67464c01 by Sergey Kozub <[email protected]>:

Add F4E2M1FN and F8E8M0FNU types

Merging this change closes #21380

PiperOrigin-RevId: 715434229