Warn when FSDP auto-wrap policy splits tied weights (#21613)

* Add tied weight detection warning for FSDP auto-wrap policy

Detect shared parameters that would be placed in separate FSDP units
by the auto-wrap policy and emit a warning before wrapping. This turns
a cryptic RuntimeError (size mismatch) into an actionable message.

Applies to both Fabric and PyTorch Lightning FSDP strategies.

Closes #21403

* Add tests for FSDP tied weight detection warning

Cover four scenarios: tied weights across units (warns), tied weights
in same unit (no warn), no shared params (no warn), no policy (no warn).

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Co-authored-by: Deependu <deependujha21@gmail.com>