Split utils key and JSON helpers

[vmoens]

Summary

• move filesystem key encoding and robust-key helpers into tensordict._utils_key_json
• move JSON backend helpers into the same internal module
• keep tensordict.utils re-exporting the old names with __module__ == "tensordict.utils"
• add focused tests for import identity, key round trip/warnings, and JSON serialization

BC notes

• imports from tensordict.utils remain valid
• helper function module paths remain tensordict.utils
• JSON backend registration still occurs when tensordict.utils is imported

Test plan

• python -m pytest test/test_utils_key_json_split.py -q
• ulimit -n 4096 && python -m pytest test/test_tensordict.py -k "robust_key or memmap" -q
• python -m compileall -q tensordict/utils.py tensordict/_utils_key_json.py
• import smoke test for _encode_key_for_filesystem and json_dumps module paths

[github-actions]

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[Compile]	Compile	[Compile] Fix torch.compile issue
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[BC-breaking] or [BC]	BC-breaking	[BC-breaking] Remove deprecated API
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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 utils key and JSON helpers

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.