apply_chat_template: consistent behaviour for return_assistant_tokens_mask=True return_tensors=True

[mrsndmn]

What does this PR do?

This PR fixes the simultaneous use of flags return_assistant_tokens_mask, return_tensors, in apply_chat_template method.

Is related to #28950 and corresponding PR #30650

Before fix

output = tokenizer.apply_chat_template(
    conversations,
    tokenize=True,
    padding=True,
    return_dict=True,
    return_assistant_tokens_mask=True,
    return_tensors='pt',
)

assert isinstance(output['attention_mask'], torch.Tensor)  # ok
assert isinstance(output['assistant_masks'], torch.Tensor) # not ok: type of `output['assistant_masks']` is `List`

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