fix(conversion): Fix size mismatch error during TF->PT model loading

[arjunaskykok]

What does this PR do?

Loading a PyTorch model from a saved TensorFlow checkpoint using
from_pretrained(..., from_tf=True) could fail with a
RuntimeError: size mismatch. The error indicated that weights like
position embeddings were expected to have the shape of word embeddings
(e.g., [vocab_size, hidden_size]).

This issue was triggered by recent changes that defaulted to initializing
the PyTorch model with meta tensors (init_empty_weights) during this
conversion process.

The root cause was in the tied weight handling logic within
load_tf2_state_dict_in_pytorch_model in modeling_tf_pytorch_utils.py.
Multiple distinct parameters initialized as meta tensors can share the same
data_ptr() == 0. The existing logic incorrectly identified these as tied
weights and reused the tensor loaded for the first parameter encountered
with data_ptr() == 0 (often the word embeddings) for subsequent parameters
that also had data_ptr() == 0.

This fix modifies the tied weight check to explicitly skip cases where
pt_weight.data_ptr() == 0, preventing the incorrect reuse of tensors
for distinct meta parameters and resolving the size mismatch error.

Includes a unit test in test_modeling_utils.py to specifically verify
this scenario using from_pretrained(..., from_tf=True) with meta initialization.

Fixes #37786

Who can review?

@Rocketknight1 @gante

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[gante]

LGTM! Thank you for pinning the issue and fixing it 🤗

[gante]

@arjunaskykok to make our CI happy:

• run make fixup on your terminal, inside the transformers root folder (this runs automated code formatting)
• commit the changes

[arjunaskykok]

@gante Will do! But need more time, since I'm on Windows and using 'make' is a bit tricky on Windows.

& 'C:\Program Files (x86)\GnuWin32\bin\make.exe' fixup
-n was unexpected at this time.
make: *** [modified_only_fixup] Error 255

[manueldeprada]

Thanks a lot @arjunaskykok !! Also LGTM and now the CI is happy after merging recent main changes. Merging this!

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