Make a copy of the torchtext utils to remove dependency

[agunapal]

Description

This PR removes the dependency of TorchServe's TextClassifier Handler on TorchText by copying the functions that are being used

Fixes #(issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Feature/Issue validation/testing

CI & Regression Pass which use TextClassifier

Checklist:

• Did you have fun?
• Have you added tests that prove your fix is effective or that this feature works?
• Has code been commented, particularly in hard-to-understand areas?
• Have you made corresponding changes to the documentation?

[mreso]

We should also consider removing https://github.com/pytorch/serve/tree/master/examples/text_classification_with_scriptable_tokenizer as it depends on torchtext as well.

[agunapal]

https://github.com/pytorch/serve/tree/master/examples/text_classification_with_scriptable_tokenizer
@mreso
Shall we remove this or update the instructions to include pip install torchtext and give a deprecation message at the top of the readme ?

[mreso]

LGTM, please address comment before merging