[bazel] Add --config=shell for easier debugging (#79350)
Bazel quality of life improvement.
This change adds a new option `--config=shell` which allows you to drop-in the shell right before the `bazel run` command is executed. For example you will have ability to examine bazel sandbox this way, run things under gdb instead of a normal run and so on.
Example usage:
```
bazel run --config=shell //:init_test
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79350
Approved by: https://github.com/dagitses
