SerializedEmbedding: override default freeze=True on deserialization

[kundaMwiza]

What does this PR do?

The default behaviour of nn.Embedding.from_pretrained is to freeze the constructed embedding weights. This may not be what the user intends to do. This PR overrides the default value and allows the user to optionally change the behaviour (for example in pipelined_model_cls.deparallelize())

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.