Add some methods on Sequential container

[Ruhaan838]

What does this PR do?

This PR adds new methods in the Sequential container that exists in flax/nnx/helpers.py methods like

• __add__ -> add two Sequential container. similar __iadd__
• __mul__ -> multiply the current Sequential container with the number and return a new container. similar __rmul__, __imul__
• __iter__ , __getitem__, __setitem__, __delitem__, __len__ same and useful.
• other list methods like append, extend, pop, and insert.

Checklist

• This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other
  checks if that's the case).
• The documentation and docstrings adhere to the
  documentation guidelines.
• This change includes necessary high-coverage tests.
  (No quality testing = no merge!)

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[Ruhaan838]

@cgarciae can you review this PR and merge this?