[Enhance] Support configure enviroment in config.

[HAOCHENYE]

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Motivation

Support config torch enviroment in config by the code block below:

# Define the config
cfg = ...
cfg.env_cfg.torch_cfg = torch_cfg = dict(
    backends.cuda.matmul.allow_tf32=True,
    torch.backends.cuda.matmul.allow_fp16_reduced_precision_reduction=True
)

After building the runner, torch.backends.cuda.matmul.allow_tf32 and torch.backends.cuda.matmul.allow_fp16_reduced_precision_reduction are set to True

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[C1rN09]

I have to say that eval/exec might make it unsafe. Besides, users might abuse it... Maybe we should consider twice on this?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

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