Use tiny models for get_pretrained_model in TFEncoderDecoderModelTest
# What does this PR do?

Use tiny models for `get_pretrained_model` in `TFEncoderDecoderModelTest`.

This is originally for avoiding **GPU OOM** for `TFRembertEncoderDecoderModelTest` on CI daily testing.
But @patrickvonplaten suggests that we should actually use the small model in the following quote:

_... think we can rename it to test_model_save_loaf_from_pretrained(...) :wink: I think this "real" name was propragated since the first encoder-decoder tests existed in PyTorch. **Since the test does no integration testing** (e.g. checking if the output corresponds to something reasonable) **it makes 0 difference whether we use dummy weights or no** dummy weights here ..._
_The documentation is not available anymore as the PR was closed or merged._