[ci, doc] fix: fix transformers version dependency on Ascend NPU (#2291)

### What does this PR do?

Flash Attention2 in `transformers==4.53.0` is not work on Ascend NPU due
to [PR line
here](3457e8e73e/src/transformers/modeling_flash_attention_utils.py (L109C5-L109C23))
in `transformers`.

In order to not affect `e2e_ascend` CI, we have to set
`transformers==4.52.4` for Ascend NPU situation by force now.

Corresponding bugfix in `transformers` will be conducted as soon as
possible, after newer transformers version containing bugfix released,
we will update the transformers version dependency in verl again.

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### Specific Changes

`transformers` version in `requirement-npu.txt` and document

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requirements-npu.txt

@@ -11,7 +11,7 @@ pyarrow>=15.0.0
 pybind11
 pylatexenc
 tensordict<=0.6.2
-transformers>=4.52.4
+transformers==4.52.4
 ray==2.46.0
 wandb
 mathruler