feat(nn): add configurable norm_layer to Mlp

[Thabhelo]

PhysicsNeMo Pull Request

Description

Adds a norm_layer parameter to physicsnemo.nn.Mlp so normalization type is configurable instead of being limited to BatchNorm1d via use_batchnorm.

Supported options:

• norm_layer="batchnorm", "layernorm", or "te_layernorm"
• norm_layer=nn.LayerNorm or get_layer_norm_class() for TE-aware LayerNorm
• Any callable factory invoked as norm_layer(out_features)

use_batchnorm=True remains backward compatible and is mutually exclusive with norm_layer.

Closes #1451

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The CHANGELOG.md is up to date with these changes.
• An issue is linked to this pull request.
• If I am implementing a new model or modifying any existing model, I have followed the Models Implementation Coding Standards. (N/A: physicsnemo.nn layer change, not physicsnemo.models.)

Notes

• norm_layer is documented in the Mlp class docstring and doctest examples; Sphinx exposes it via docs/api/nn/layers/fully_connected_mlp.rst automodule.

Dependencies

None.

Test plan

• pytest test/nn/module/test_mlp_layers.py -v
• pytest --doctest-modules physicsnemo/nn/module/mlp_layers.py
• pre-commit hooks on changed files (ruff, interrogate, markdownlint, license, import-linter)

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[greptile-apps]

Greptile Summary

This PR adds a norm_layer parameter to physicsnemo.nn.Mlp, replacing the hard-coded BatchNorm1d path with a flexible factory resolved by the new _resolve_norm_factory helper. The use_batchnorm flag is preserved for backward compatibility and is mutually exclusive with the new parameter.

• _resolve_norm_factory accepts a string alias (\"batchnorm\", \"layernorm\", \"te_layernorm\"), a class, or any callable factory; unknown strings raise ValueError and a pre-instantiated nn.Module is explicitly rejected.
• Both \"layernorm\" and \"te_layernorm\" resolve to exactly the same get_layer_norm_class() factory with no behavioral distinction — \"te_layernorm\" does not enforce TE availability and silently falls back to nn.LayerNorm, making the two strings undifferentiated aliases despite the docstring listing them separately.

Important Files Changed

Filename	Overview
physicsnemo/nn/module/mlp_layers.py	Adds norm_layer parameter and _resolve_norm_factory helper; "layernorm" and "te_layernorm" strings are silent aliases (both call get_layer_norm_class()), which makes the "te_layernorm" option misleading
test/nn/module/test_mlp_layers.py	Good coverage of new norm_layer paths; no test exercises the "te_layernorm" string directly to verify it differs from "layernorm"
CHANGELOG.md	Entry added under the correct section with accurate description of the new norm_layer parameter and backward-compatibility note

_{Reviews (1): Last reviewed commit: "feat(nn): add configurable norm_layer to..." | Re-trigger Greptile}

[Thabhelo]

Okay cool, I have addressed the feedback in the latest push:

• "layernorm" now maps to PyTorch nn.LayerNorm; "te_layernorm" requires transformer_engine and CUDA (raises otherwise).
• get_layer_norm_class() remains available as a user-supplied factory for TE-aware auto selection.
• self.norm_layer is stored on Mlp for introspection.

Added tests for the TE string path and the stored attribute, too.

[Thabhelo]

Greptile feedback from the first review is addressed in 8896888. Replies are on the inline threads. docs come from the updated Mlp docstrings via Sphinx automodule.

[Thabhelo]

@loliverhennigh gentle ping when you have a moment. Greptile feedback is addressed in 8896888. I will rebase onto main shortly to clear the merge conflict. Thanks!