Changes in support of github.com/openxla/stablehlo

• Moved stablehlo to its own repository.
• Updated dependencies.
• Package dtypes:
  • Added DType.Bits field indicating the number of bits in the data type -- supporting sub-byte types including
    quantized types.
  • Added DType.IsPromotableTo(target) to check if types are promotable to the target type -- it follows the StableHLO rule.
  • Cleaned up generators for dtypes package.
• Package xlabuilder:
  • Deprecated SelectAndScatterSum.