[Fix] fix train example

[teamwong111]

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Motivation

Related issue: #1470.
Some users want to test the example/train.py on single GPU even though there are multiple GPUs on their machines.
Force to use GPU:0 can avoid Error traceback.

Modification

Assign device_ids = [0] and fix a format error by the way.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
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

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[zhouzaida]

Please add the following warning information in the docstring.

mmcv/parallel/data_parallel.py

.. warning::
    MMDataParallel only supports single GPU training, if you need to train with multiple GPUs, please use MMDistributedDataParallel instead. If you have multiple GPUs and you just want to use MMDataParallel, you can set the ``CUDA_VISIBLE_DEVICES=0`` or ``MMDataParallel(model, device_ids=[0])``.