feat(nlp): migrate TF GloVe to FastText vectors

[athewsey]

Issue #, if available: #1

Description of changes:

In TF.Keras NLP notebooks:

• Migrate from GloVe 6B to pre-trained cc.en.300.vec.gz from FastText
• Tweak default instance type to ml.g4dn.xlarge
• Add some mitigations to help prevent file corruptions/overwrite when running local & SageMaker notebooks in parallel
• Update default SageMaker training container TF v1.14->1.15, in line with the currently configured SMStudio kernel

FastText offers multi-lingual pre-trained embeddings (vs English-only) and marginally faster download time (although in current implementation this is offset by increases in downstream processing times)

PyTorch alternatives not yet updated, pending further testing.

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[athewsey]

Cloned to clean environment and ran through - all seems fine