fix: default value reflects the runtime environment variables rather than the ones present at import time.
Author: junrae6454
Pull request: yes

# What does this PR do?
Refactor deterministic flag handling in Flash Attention forward method

- Set the `deterministic` flag to False by default in `_flash_attention_forward`.
- Retrieve the deterministic flag from the environment variable `FLASH_ATTENTION_DETERMINISTIC` within the function scope to recognize runtime environment variables.
- Adjusted `flash_kwargs` dictionary to use the updated deterministic flag handling mechanism.

This change ensures that the parameter's default value reflects the runtime environment variables rather than the ones present at import time.

```python
>>> import os
>>> def default_param(param = os.environ.get("deterministic", "0") == "1"):
...     print(param)
...
>>> default_param()
False
>>> os.environ["deterministic"] = "1"
>>> default_param()
False
```
```python
>>> import os
>>> os.environ["deterministic"] = "1"
>>> def default_param(param = os.environ.get("deterministic", "0") == "1"):
...     print(param)
...
>>> default_param()
True
>>> os.environ["deterministic"] = "0"
>>> default_param()
True
```

Reference: https://github.com/huggingface/transformers/pull/31961

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## Who can review?

Thanks to @ArthurZucker 

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Comments:
For the quality checks, could you rebase on main to include the upstream changes. This should fix things