use -m option to run scripts as modules

[tianyu-l]

Running python or torchrun with -m option helps resolve package structure without the need to do pip install -e.

This is suggested by @yzhangcs in #897 (comment)

It avoids the potential pitfalls of using pip install -e ., which can sometimes lead to inconvenient and hard-to-debug issues, especially when we're making frequent changes to the codebase.