Validate non-null data pointer in Tensor accessors

[rascani]

Summary:
Add null-data-pointer validation to all six data accessor methods on the Tensor class (const_data_ptr, mutable_data_ptr, and deprecated data_ptr — two overloads each).

The check fires when numel() > 0 but the underlying data_ pointer is null, which indicates an invalid tensor state. This prevents null pointer dereferences in all 225+ kernel implementations that call these accessors.

The Lionhead fuzzer (FuzzKernels harness, T234178948) has been filing 50-100+ crash tasks because it constructs tensors with non-zero numel but null data pointers. Every kernel that dereferences the result of const_data_ptr() crashes. This centralized check catches the invalid state at the accessor level, eliminating the entire class of fuzzer findings.

Tensors with numel() == 0 and a null data pointer remain valid (empty tensors).

Differential Revision: D103467785

[pytorch-bot]

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❌ 7 New Failures, 2 Unrelated Failures

As of commit dc47f83 with merge base a3dd0fa ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 14160877dfc6761e55f56c344d74c89efffb149cd1cda9ddc44939ea3a9df7c9 /exec failed with exit code 8
• pull / unittest / windows / windows-job (gh)
  Process completed with exit code 8.
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 4d460386c301a0424f72c65762fd52c9f54b3250913da00e660c3afa028590ac /exec failed with exit code 32
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 32
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 4b22369b613034bb6e84c621996056cfe76c5f8f7837a115905e3576a22064f9 /exec failed with exit code 8
• pull / unittest-editable / windows / windows-job (gh)
  Process completed with exit code 8.
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t e6e51b2dd7ae9c1f5d9eda2e58207603fb47c7d081445fb8104f70c6ae7427f6 /exec failed with exit code 1

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@rascani has exported this pull request. If you are a Meta employee, you can view the originating Diff in D103467785.

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