Set scale_embedding to False in some TF tests
# What does this PR do?

This PR set `scale_embedding=False` in `TFSpeech2TextModelTester` to avoid `inputs_embeds` and the PT/TF difference being scaled by 4 - the objective is to keep a low tolerance `1e-5` in the PT/TF test, as we see several times this is a strong safe guard!

(This is not a real bug. It's also similar to #15684, where we got larger differences between PT/TF simply because the model weights are initialized with larger values).

TF: @gante @Rocketknight1 
Speech: @patrickvonplaten 

## More context

Set `scale_embedding` to `False` in some TF tests.

Current `Speech2TextConfig` has default [scale_embedding=True](https://github.com/huggingface/transformers/blob/9932ee4b4bca9045d941af6687ef69eedcf68483/src/transformers/models/speech_to_text/configuration_speech_to_text.py#L137). Therefore we have

- [self.embed_scale = tf.math.sqrt(float(embed_dim))](https://github.com/huggingface/transformers/blob/9932ee4b4bca9045d941af6687ef69eedcf68483/src/transformers/models/speech_to_text/modeling_tf_speech_to_text.py#L742).
- The tests for `speech_to_text` has [hidden_size=16](https://github.com/huggingface/transformers/blob/9932ee4b4bca9045d941af6687ef69eedcf68483/tests/speech_to_text/test_modeling_tf_speech_to_text.py#L75), and therefore `inputs_embeds` will be scaled by 4.

https://github.com/huggingface/transformers/blob/9932ee4b4bca9045d941af6687ef69eedcf68483/src/transformers/models/speech_to_text/modeling_tf_speech_to_text.py#L843-L844

Since `inputs_embeds` here is obtained by some (conv.) layer instead of via look-up table, it contains some tiny difference between PT/TF. This difference is scaled by 4 through `self.embed_scale`.

This makes `TFSpeech2TextModel` **the only one** model that will fail the aggressive PT/TF test introduced in #15839 (with low tolerance `1e-5`). More precisely, the output tensors failed are `encoder_hidden_states_0` and `encoder_hidden_states_1`.

## Results

**With this PR, the tolerance `1e-5` works for all TF models' `test_pt_tf_model_equivalence` (in #15839), both on GPU / CPU, tested 100 times per model.**
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15952). All of your documentation changes will be reflected on that endpoint.