[TF:XLA] Add a Cholesky decomposition implementation.

Currently the implementation is fully unrolled, which can cause code size blowups at large matrix sizes. We can explore reducing code size in a subsequent change.

Create a new directory tensorflow/compiler/tf2xla/lib of XLA utility functions. Move batch matmul implementation into the utility directory. Add helpers for batch matmul, triangular solve, and Cholesky decomposition.

PiperOrigin-RevId: 175338698