Fix ViTMAEModelTester
# What does this PR do?

Current `ViTMAE` test has `(TF)ViTMAEModelTester` not setting `decoder` configs, like `decoder_hidden_size`, `decoder_intermediate_size` etc.

For the model class `(TF)ViTMAEForPretraining`, the default values for these attributes in `ViTMAEConfig` are used, and cause some tests being slow and consuming quite a lot of memory.

This PR fixes it.

(This might be `one` of the reasons where sometimes CI tests get `node down: Not properly terminated` -> `replacing crashed worker gw7` -> hang forever -> timeout)
_The documentation is not available anymore as the PR was closed or merged._