Add configurable checkpoint filenames and state-dict key remapping

[Thabhelo]

Summary

• Add optional filename_format to save_checkpoint so model checkpoint basenames can use custom layouts (e.g. zero-padded epochs) while preserving legacy naming when unset.
• Add Module._backward_compat_state_dict_mapper and apply it in from_checkpoint before load_state_dict, mirroring the existing constructor-arg backward-compat hook for refactored parameter names.

Closes #1175
Closes #1173

Test plan

• pytest test/utils/test_checkpoint.py::test_save_checkpoint_filename_format
• pytest test/utils/test_checkpoint.py::test_save_checkpoint_filename_format_invalid_placeholder
• pytest test/models/test_from_checkpoint.py::test_from_checkpoint_state_dict_mapper
• pre-commit on changed files

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[greptile-apps]

Greptile Summary

This PR adds two independent utilities: a filename_format parameter to save_checkpoint for custom model-checkpoint filenames (e.g., zero-padded epochs), and a _backward_compat_state_dict_mapper hook on Module for version-aware state-dict key remapping in from_checkpoint.

• The state-dict mapper feature (module.py) is well-structured and correctly integrated into both the zip and tar from_checkpoint code paths using the existing mdlus_file_version metadata key.
• The filename_format feature (checkpoint.py) works correctly when epoch is explicitly specified, but _resolve_checkpoint_index uses a legacy regex that cannot discover custom-formatted filenames; auto-indexing (epoch=None with a custom format) will always resolve to index 0 and silently overwrite the first checkpoint on every subsequent save.
• _resolve_checkpoint_index is invoked unconditionally but its result is ignored on the legacy (filename_format=None) code path, triggering a redundant filesystem scan on every call.

Important Files Changed

Filename	Overview
physicsnemo/utils/checkpoint.py	Introduces _resolve_checkpoint_index helper and filename_format parameter; auto-indexing is broken when filename_format is used without an explicit epoch because the helper's regex only matches legacy filenames, and the helper result is unused on the legacy code path causing a double scan.
physicsnemo/core/module.py	Adds _backward_compat_state_dict_mapper classmethod (no-op base) and _apply_backward_compat_state_dict static method; correctly wired into both zip and tar from_checkpoint paths using the existing mdlus_file_version metadata key.
test/utils/test_checkpoint.py	Adds two tests for filename_format: happy-path with zero-padded epoch and invalid-placeholder error. Tests always pass an explicit epoch, so the broken auto-index path is not exercised.
test/models/test_from_checkpoint.py	Adds end-to-end test for state-dict key remapping across model versions; covers key rename and verifies weight values are preserved.
CHANGELOG.md	Adds changelog entries for both new features under the Unreleased section.

_{Reviews (1): Last reviewed commit: "Add checkpoint filename_format and state..." | Re-trigger Greptile}

[Thabhelo]

Greptile triage for 8e60df5:

• P1 auto-indexing with custom filename_format: fixed. _filename_format_index_pattern parses formatted basenames, and test_save_checkpoint_filename_format_auto_index covers the epoch=None path.
• P2 no-op ternary: fixed. plain return 0 now.
• P2 redundant legacy scan: fixed. legacy naming reuses resolved_index directly.