Add a flag to override the GEMM autotuner with a specified textproto configuration.
Add a flag to override the GEMM autotuner with a specified textproto configuration.

This makes it easier to debug issues with Triton, since [persisted autotuning](https://openxla.org/xla/persisted_autotuning) does not offer a workflow that would be efficient enough:

- `--xla_gpu_dump_autotune_results_to=` doesn't work if the autotuner crashes, so we would need to construct the cache manually from the logs; furthermore, the cache only applies to a specific HW, so we can't use it to override the autotuner on a different GPU
- `--xla_gpu_load_autotune_results_from=` reads a file so we need to copy the file into the source tree, and modify the BUILD file to include it into the build; with this new flag, we can just copy the flag from the VLOG without having to fiddle around with any files
