Restructure Pyconfig

[A9isha]

Description

Restructure pyconfig.py such that we can create multiple configs. This becomes important if we have a need from an RL workload to have training and inference configs which differ in sharding strategies among others.
We also now use OmegaConfig instead of yaml.safe_load and get its benefits

FIXES: b/396747968

Tests

Tested this code with Llama2-7b on v4-8

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.

[shralex]

if you'd like to add a unit test, you could for example test that invoking initialize twice creates two different configs, and that attempts to modify a config will result in an error

[A9isha]

if you'd like to add a unit test, you could for example test that invoking initialize twice creates two different configs, and that attempts to modify a config will result in an error

Done - thank you Alex!

[khatwanimohit]

LGTM

[gobbleturk]

Awesome work, thanks Anisha!