Add a metric for the MATH dataset (competition_math).
I believe the only failed test related to this PR is tests/test_metric_common.py::LocalMetricTest::test_load_metric_competition_math. It gives the following error:

ImportError: To be able to use this dataset, you need to install the following dependencies['math_equivalence'] using 'pip install git+https://github.com/hendrycks/math.git' for instance'

It fails along with (these fail with ImportError as well):
test_load_metric_bertscore
test_load_metric_bleurt
test_load_metric_comet
test_load_metric_coval

Let me know if there is anything I need to change. Hi ! The script looks all good thanks :)

To fix the CI you just need to merge `master` into your branch
```
git fetch upstream/master
git merge upstream/master
```

Then you also need to add `math_equivalence` to the list of git packages installed for the tests in `additional-tests-requirements.txt`
https://github.com/huggingface/datasets/blob/ba831e4bcd175ae3d52afbf7d12c4f625bf541b0/additional-tests-requirements.txt#L1-L3 I ran:

git fetch upstream
git merge upstream/master

And I also added  math_equivalence to the list of git packages installed for the tests in additional-tests-requirements.txt

tests/test_metric_common.py fails with the same errors as before. tests/test_dataset_cards.py also fails, but it doesn't look related to this PR (it's an issue datasets/ami/README.md). @lhoestq Anything else I can do? I re-merged again and am getting the same test failures as described in the previous comment.
This metric computes accuracy for the MATH dataset (https://arxiv.org/abs/2103.03874) after canonicalizing the prediction and the reference (e.g., converting "1/2" to "\\\\frac{1}{2}").