[nnx] improve graph_utils._set_key_tuple

[cgarciae]

What does this PR do?

• Checks if the position being updated is the same as the exiting one and returns instead of raising an error. This allows Module to use tuples in lifted transforms.
• Includes some ruff formatting updates.

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (85dfad2) 56.31% compared to head (3fcc013) 56.30%.

Files	Patch %	Lines
flax/experimental/nnx/nnx/graph_utils.py	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3592      +/-   ##
==========================================
- Coverage   56.31%   56.30%   -0.02%     
==========================================
  Files         100      100              
  Lines       11952    11955       +3     
==========================================
  Hits         6731     6731              
- Misses       5221     5224       +3     

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