fix flaky m2m100 test
# What does this PR do?

The `test_retain_grad_hidden_states_attentions` test is sometimes failing for `M2M100`, with the error
` AttributeError: 'NoneType' object has no attribute 'retain_grad'`

 This is because of `layerdrop` sometimes a layer is skipped and the `encoder_attenion/decoder_attentions/cross_attentions` can be `None`.

This PR sets the `config.encoder_layerdrop` and `config.decoder_layerdrop` to 0  in tests to make the tests deterministic.