Store tokenizer metadata/object within model

[SeanNaren]

When a model is saved, we do not store information pertaining to the tokenizer. This means we require the tokenizer to be re-created and assigned like below at inference/test time:

model = LitAutoModelTransformer.load_from_checkpoint('checkpoint.pt')
tokenizer = AutoTokenizer.from_pretrained('bert-base-cased')
model.tokenizer = tokenizer
...

It would be preferred that after specifying the tokenizer at training time, inference knows which tokenizer to use.