Remove NLTK processors

[hunterhector]

This PR fixes #455.

Description of changes

1. NLTK dependencies in the core Forte folders are removed.
2. NLTK imports from examples and tests are both replaced with the forte-wrapper version
3. Installed forte-wrapper with the NLTK dependencies to conduct the tests.

Possible influences of this PR.

Describe what are the possible side-effects of the code change.

Test Conducted

Describe what test cases are included for the PR.

[codecov]

Codecov Report

Merging #466 (5147fd4) into master (70eabab) will decrease coverage by 0.46%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #466      +/-   ##
==========================================
- Coverage   81.11%   80.64%   -0.47%     
==========================================
  Files         209      209              
  Lines       14479    14477       -2     
==========================================
- Hits        11744    11675      -69     
- Misses       2735     2802      +67     

Impacted Files	Coverage Δ
forte/processors/third_party/__init__.py	100.00% <ø> (ø)
...rs/base/data_augment_replacement_processor_test.py	99.42% <100.00%> (ø)
...ors/data_augment/algorithms/eda_processors_test.py	98.03% <100.00%> (ø)
tests/forte/processors/nltk_processors_test.py	99.05% <100.00%> (ø)
...e/processors/pretrained_encoder_processors_test.py	29.16% <100.00%> (ø)
tests/forte/processors/writer_test.py	97.22% <100.00%> (-0.08%)	⬇️
forte/processors/third_party/nltk_processors.py	32.32% <0.00%> (-67.68%)	⬇️

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