removed nnx dataclass

[chiamp]

Removed nnx dataclass, after #3720.

Instead of:

@nnx.dataclass
class Foo(nnx.Pytree):
  a: int = nnx.treenode_field()
  b: int = nnx.field()

users should now do:

@dataclasses.dataclass
class Foo(nnx.Pytree):
  a: nnx.TreeNode[int]
  b: int = dataclasses.field()

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.52%. Comparing base (e4282ee) to head (d4ac6c3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3742      +/-   ##
==========================================
- Coverage   58.67%   58.52%   -0.15%     
==========================================
  Files         103      102       -1     
  Lines       12437    12376      -61     
==========================================
- Hits         7297     7243      -54     
+ Misses       5140     5133       -7     

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