[XLA] FP16 Dot support for the CPU and GPU backends.

Extend the stream interface ThenBlasGemmWithAlgorithm to support F16 matrix
multiplication with computation type FP32.

Extend the stream executor interface DoBlasGemmWithAlgorithm to support F16
GEMM with computation type FP32.

Extend the CPU IR emitter to handle F16 Dot instruction, and add F16 matrix
multiplication implementation to the CPU runtime.

Extend the GPU backend to handle FP16 GEMM Thunk.

Replicate the existing matrix multiplication test cases in
matrix_ops_simple_test and dot_operation_test for FP16.

RELNOTES:
PiperOrigin-RevId: 187369731