Fix stuff related to the causal_mask in CodeGen.
Author: GeneZC
Pull request: yes

1. Line 613, `_keys_to_ignore_on_load_missing  =  [r"h\.\d+\.attn\.masked_bias", r"h\.\d+\.attn\.bias"]` => `_keys_to_ignore_on_load_missing  =  [r"h\.\d+\.attn\.causal_mask"]` to load correctly from CodeGen checkpoint without `causal_mask`.
2. Line 152, `causal_mask = self.causal_mask[:, :, key_length - query_length : key_length, :key_length]
` => `causal_mask = self.causal_mask[:, :, key_length - query_length : key_length, :key_length].bool()
` to alleviate potential user warning saying like `UserWarning: where received a uint8 condition tensor. This behavior is deprecated and will be removed in a future version of PyTorch. Use a boolean condition instead.`.

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

cc @ArthurZucker and @younesbelkada 

Following up on my comment, here is a script I ran: 

```python

import torch
import torch.nn as nn

class DummyModelUint(nn.Module):
    def __init__(self):
        super().__init__()
        self.linear = nn.Linear(10, 10)
        self.register_buffer("attention_mask", torch.ones(10, 10, dtype=torch.uint8))

    def forward(self, x):
        return self.linear(x)

class DummyModelBool(nn.Module):
    def __init__(self):
        super().__init__()
        self.linear = nn.Linear(10, 10)
        self.register_buffer("attention_mask", torch.ones(10, 10, dtype=torch.bool))

    def forward(self, x):
        return self.linear(x)

model_uint8 = DummyModelUint()
state_dict = model_uint8.state_dict()

model_bool = DummyModelBool()
model_bool.load_state_dict(state_dict)

print(model_bool.attention_mask.dtype)
>>> torch.bool
```
So even if you save `uint8` buffers on the state dict, if you load it through a model that has `torch.bool` buffer it will overwrite it in `torch.bool`. So I don't think the first fix is needed if we merge #21384 ! 
However your second fix might make sense, just need more clarification ;) 

@younesbelkada 
Thanks for your review.

1. Removing `[r"h\.\d+\.attn\.masked_bias", r"h\.\d+\.attn\.bias"]` basically due to there are not `masked_bias` or `bias` in `attn`. I conjecture the reason why they leave these two terms here is that they copy part of the code from GPTJ (https://github.com/huggingface/transformers/blob/main/src/transformers/models/gptj/modeling_gptj.py#L723).
2. Introducing `.bool()` due to it will raise a warning when later execute the `torch.where` (https://github.com/huggingface/transformers/blob/main/src/transformers/models/codegen/modeling_codegen.py#L165). However, it seems that this will be addressed in another PR.


2. The causal mask should not be a `bool` no?  I might not be looking at the correct line in the code, but the only bool is going the be the attention mask that we feed the network, which is then used to created the causal mask by multiplying with `torch.finfo(...).min`. (so something like -1e-9)

I agree that we should leave the fix 2 to PR https://github.com/huggingface/transformers/pull/21384.