[FEA]: PreprocessLogParsingStage should be folded into PreprocessNLPStage

[dagardner-nv]

Is this a new feature, an improvement, or a change to existing functionality?

Improvement

How would you describe the priority of this feature request

Medium

Please provide a clear description of problem this feature solves

The pre-processing stage in examples/log_parsing is 99% similar to PreprocessNLPStage and should either be a subclass or PreprocessNLPStage should optionally be configured to add white-space around punctuation marks.

Describe your ideal solution

n/a

Describe any alternatives you have considered

No response

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct
• I have searched the open feature requests and have found no duplicates for this feature request

[dagardner-nv]

Folding this into the PreprocessNLPStage has the advantage that it enables C++ execution for the stage, subclassing has the advantage of providing an example on how to extend one of the built-in Morpheus stages.

Since pre-processing is likely the most common custom class for users to want to implement I'm going to go with the subclass route.

[dagardner-nv]

Turns out PreprocessLogParsingStage isn't needed at all. The only thing it does is provide different default constructor values, along with surrounding punctuation with white-space:

        for symbol in string.punctuation:
            text_ser = text_ser.str.replace(symbol, ' ' + symbol + ' ')

However this isn't needed as the cudf subword tokenizer removes punctuation, and the results from both stages are identical.

From:
https://github.com/rapidsai/cudf/blob/43f0c3d70cb8bf777d6856c432245faaa947cc4f/cpp/include/nvtext/subword_tokenize.hpp

 * The strings are first normalized by converting to lower-case, removing
 * punctuation, replacing a select set of multi-byte characters and
 * whitespace characters.