[BUG] CausalLanguageModeling masking error on last item only condition

[sungho-ham]

Description

When using clm for masking, it generates wrong masking schema.
It can be checked by following simple code.
In case of length 2 input, there should not be difference between last item only and all items conditions.
However, there is difference between them.

import torch
from transformers4rec.torch import masking

def get_masking_info(train_on_last:bool):
    item_ids = torch.tensor([[1, 2, 0], ])
    mask = masking.CausalLanguageModeling(hidden_size=10, train_on_last_item_seq_only=train_on_last)
    masking_info = mask.compute_masked_targets(item_ids, training=True)
    return masking_info

print(get_masking_info(False))
print(get_masking_info(True))

MaskingInfo(schema=tensor([[ True, False, False]]), targets=tensor([[2, 0, 0]]))
MaskingInfo(schema=tensor([[ True,  True, False]]), targets=tensor([[2, 0, 0]]))  -> schema shoule be [ True,  True, False]

Related Code

Transformers4Rec/transformers4rec/torch/masking.py

Line 298 in 348c963

mask_labels = item_ids != self.padding_idx

I think following code is correct:
mask_labels = labels != self.padding_idx

[sungho-ham]

I've found my suggestion was wrong and already fixed.
#723 (review)
However, from what I understand, the results is not correct. I reported it again according to the bug reporting format.