Use get_vocab method instead of vocab

[butsugiri]

What does this PR do ?

This PR fixes the issue of missing vocab property for NeMo AutoTokenizer.

Currently, NeMo's AutoTokenizer obtains a list of vocabulary from tokenizer.vocab.
However, it can be problematic for the following reasons:

• the property is sometimes missing for certain type of HuggingFace tokenizers.
• the property is not documented in Huggingface Transformers Library.

For example, non-fast version of T5Tokenizer does not seem to have vocab as follows:

In [8]: assert getattr(AutoTokenizer.from_pretrained('google-t5/t5-base'), "vocab") # This one is OK
In [9]: assert getattr(AutoTokenizer.from_pretrained('google-t5/t5-base', use_fast=False), "vocab") # Not OK
…
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[9], line 1
----> 1 assert getattr(AutoTokenizer.from_pretrained('google-t5/t5-base', use_fast=False), "vocab")

AttributeError: 'T5Tokenizer' object has no attribute 'vocab'

This can be a problem for GPTSFTChatDataset because it assumes that a given tokenizer has avocab property.

NeMo/nemo/collections/nlp/data/language_modeling/megatron/gpt_sft_chat_dataset.py

Line 313 in 7d3d9ac

assert hasattr(self.tokenizer, "vocab"), "tokenizer should have vocab property, not supported"

This PR renames a reference to vocabproperty to get_vocab method.
I believe that this method also returns a list of vocabulary in a same manner as vocab property.
Thus, existing tokenizers would not be affected.

Collection: [Note which collection this PR will affect]

This PR may affect tokenizer component.

Changelog

renamed vocab to get_vocab

Usage

n/a

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[butsugiri]

@ericharper @titu1994 @blisc @okuchaiev Would you please review my PR?