Simplify CLI args validation and ensure CLI args take precedence over config file.

[Iain-S]

What does this PR do?

• We simplify the processsing of config file sub-sections (e.g. fsdp, megatron, ipex, deepspeed, etc). Previously, any time a dict was encountered in a config, all dicts in the config were re-processed.
• CLI options such as --megatron_some_option will no longer be overridden by config options (there was previously no if getattr(args, name, None) check before overwriting the arg.name). It makes things more consistent but note that it does change existing behaviour.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Issues

Somewhat related to #2731.

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@muellerzr

Notes for Reviewers

I've made the logic in _validate_launch_command() more explicit as the proposed

    if args.config_file is not None or (os.path.isfile(default_config_file) and not args.cpu):

is equivalent to the previous

    if args.config_file is not None or os.path.isfile(default_config_file) and not args.cpu:

However, if what we really want is

    if (args.config_file is not None or os.path.isfile(default_config_file)) and not args.cpu:

then I'll happily edit but note that it would be changing the existing behaviour.

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[muellerzr]

Nice refactor and tests! Thanks for doing this 🤗

[muellerzr]

FYI this broke FSDP as defaults for that are not always None. Making a follow-up

[Iain-S]

Let me know if I need to resubmit this PR. If you could suggest a test case to catch the problem you found, that would be great.