[nnx] Add NNX WeightNorm.

[nilq]

What does this PR do?

This PR addresses #4426 by implementing an NNX WeightNorm class. This brings the requested normalization class to the NNX library – implemented to be consistent with the corresponding module in Linen.

Checklist

• This change is discussed in NNX WeightNorm #4426.
• The documentation and docstrings adhere to the
  documentation guidelines.
• This change includes necessary high-coverage tests.

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[cgarciae]

We should use the same Filter supported by nnx.filterlib which are a bit more general and used thought the library.

[cgarciae]

To resolve the pytest issues you will have to add some deprecation warning to the list here:

flax/pyproject.toml

Lines 186 to 188 in 6af8fcb

	# DeprecationWarning: numpy.core is deprecated
	"ignore:.*numpy.core is deprecated.*:DeprecationWarning",
	]

If this is not from your PR we can fix it in main first.