[Refactor] Split TensorDict helper functions

[vmoens]

Summary

• move top-level tensordict._td helper functions into tensordict._td_functions
• keep tensordict._td as the compatibility facade for these helpers
• add import-identity coverage for the moved helpers and top-level tensordict exports

BC notes

• existing imports from tensordict._td remain valid
• top-level tensordict helper exports still resolve to the same function objects
• moved helpers keep __module__ == "tensordict._td"

Test plan

• python -m pytest test/test_compatibility.py::test_td_helper_import_paths_are_preserved -q
• ulimit -n 4096 && python -m pytest test/test_tensordict.py -k "from_module or from_modules or from_pytree or from_consolidated or save_load_memmap or maybe_dense_stack" -q
• python -m pytest test/test_compatibility.py -q -k "not redis and not StoreFromSchema and not Preallocation"
• python -m compileall -q tensordict/_td.py tensordict/_td_functions.py

The local file descriptor limit is 256, so the memmap-heavy targeted test selection needs ulimit -n 4096 locally.

[github-actions]

PR Title Label Error

PR title must start with a label prefix in brackets (e.g., [BugFix]).

Current title: Split TensorDict helper functions

Supported Prefixes

Your PR title must start with exactly one of these prefixes (case-insensitive):

Prefix	Label Applied	Example
[BugFix] or [Fix]	bug	[BugFix] Fix memory leak in TensorDict
[Feature]	Feature	[Feature] Add new storage backend
[Doc] or [Docs]	documentation	[Doc] Update installation guide
[Refactor]	Refactor	[Refactor] Clean up module imports
[CI]	CI	[CI] Fix workflow permissions
[Test] or [Tests]	Test	[Test] Add unit tests for nn module
[Compile]	Compile	[Compile] Fix torch.compile issue
[Performance] or [Perf]	Performance	[Perf] Optimize tensor operations
[Deprecation]	Deprecation	[Deprecation] Mark old function
[Setup]	setup	[Setup] Update build configuration
[Distributed] or [Dist]	Distributed	[Distributed] Add scatter collective
[Benchmark] or [Bench]	Benchmarks	[Benchmark] Add compile benchmark
[Typing] or [Type]	Typing	[Typing] Add type stubs
[BC-breaking] or [BC]	BC-breaking	[BC-breaking] Remove deprecated API
[Formatting] or [Format]	Formatting	[Format] Fix code style
[Quality]	Quality	[Quality] Improve error messages

Note: Matching is case-insensitive. Common variations (singular/plural) are supported.

[github-actions]

PR Title Label Error

PR title must start with a label prefix in brackets (e.g., [BugFix]).

Current title: Split TensorDict helper functions

Supported Prefixes

Your PR title must start with exactly one of these prefixes (case-insensitive):

Prefix	Label Applied	Example
[BugFix] or [Fix]	bug	[BugFix] Fix memory leak in TensorDict
[Feature]	Feature	[Feature] Add new storage backend
[Doc] or [Docs]	documentation	[Doc] Update installation guide
[Refactor]	Refactor	[Refactor] Clean up module imports
[CI]	CI	[CI] Fix workflow permissions
[Test] or [Tests]	Test	[Test] Add unit tests for nn module
[Compile]	Compile	[Compile] Fix torch.compile issue
[Performance] or [Perf]	Performance	[Perf] Optimize tensor operations
[Deprecation]	Deprecation	[Deprecation] Mark old function
[Setup]	setup	[Setup] Update build configuration
[Distributed] or [Dist]	Distributed	[Distributed] Add scatter collective
[Benchmark] or [Bench]	Benchmarks	[Benchmark] Add compile benchmark
[Typing] or [Type]	Typing	[Typing] Add type stubs
[BC-breaking] or [BC]	BC-breaking	[BC-breaking] Remove deprecated API
[Formatting] or [Format]	Formatting	[Format] Fix code style
[Quality]	Quality	[Quality] Improve error messages

Note: Matching is case-insensitive. Common variations (singular/plural) are supported.