Fix the serialization of SequenceSummary block

[sararb]

Saving a transformer-based model with SequenceSummary as a post block was throwing an error:

E       NotImplementedError: 
E       Layer SequenceMean has arguments ['self', 'initializer_range']
E       in `__init__` and therefore must override `get_config()`.
E       
E       Example:

Goals ⚽

• Add a get_config method to the SequenceSummary block

Testing Details 🔍

• Update the test test_transformer_encoder_with_post to check the serialization of the transformer block defined with a SequenceSummary post block.

[github-actions]

Documentation preview

https://nvidia-merlin.github.io/models/review/pr-927

[rnyak]

fixes the model saving error in #908 due to SequenceSummary as a post block.

[rnyak]

rerun tests

[sararb]

rerun tests

[sararb]

rerun tests

[oliverholworthy]

we may also need to also save the summary argument from the __init__ in this config, and implement from_config to restore the layer to the same state correctly

[sararb]

You're right, I have updated the SequenceSummary to make it more flexible.
The reason I didn't add from_config is that SequenceSummary was used as a base class of the transform blocks SequenceLast, SequenceFirst, SequenceMean, and SequenceClsIndex where the summary parameter was set to a default value for each block.