Preserve spaces in GPT-2 tokenizers (#2778)
* Preserve spaces in GPT-2 tokenizers
Preserves spaces after special tokens in GPT-2 and inhereted (RoBERTa)
tokenizers, enabling correct BPE encoding. Automatically inserts a space
in front of first token in encode function when adding special tokens.
* Add tokenization preprocessing method
* Add framework argument to pipeline factory
Also fixes pipeline test issue. Each test input now treated as a
distinct sequence.
