[Refactor] Unify the --out and --dump in tools/test.py.

[mzr1996]

Motivation

The --out of tools/test.py in MMClassification and MMDetection are different. And users may be confused about the difference between --out and --dump.

Modification

Remove the --dump argument and use --out argument to output the result file.
Add a --out-item argument to specify the content of the result file.

BC-breaking

The --dump argument is not available now, and please change it to --out.
And the original --out can be replaced by --out OUT --out-item metrics

Output model predictions

# Original
python tools/test.py CONFIG CHECKPOINT --dump results.pkl

# Now
python tools/test.py CONFIG CHECKPOINT --out results.pkl

Output evaluation metrics

# Original
python tools/test.py CONFIG CHECKPOINT --out results.pkl

# Now
python tools/test.py CONFIG CHECKPOINT --out results.pkl --out-item metrics

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

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

[Ezra-Yu]

LGTM.