remove unnecessary guard code related with pytorch versions 1.4.2 ~ 1.7.0
Author: ji-huazhong
Pull request: yes


# What does this PR do?

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If I understand correctly, the minimum supported version of PyTorch for transformers is now 1.11.0. Therefore, this PR removes unnecessary guard code. 
The relevant PR is as follows：
- https://github.com/huggingface/transformers/pull/28207
- https://github.com/huggingface/transformers/pull/8979




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cc @ydshieh


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