llama : fix Roberta embeddings (#10856)

llama : fix Roberta embeddings (#10856)

* fix: Use gpt2 tokenizer for roberta and add eos/bos tokens

Branch: RobertaTokenizer

Signed-off-by: Gabe Goodhart <redacted>
* fixes to position embeddings

Signed-off-by: Sukriti-Sharma4 <redacted>
* map roberta-bpe to gpt-2

Signed-off-by: Sukriti-Sharma4 <redacted>
* fix linting

Signed-off-by: Sukriti-Sharma4 <redacted>
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Signed-off-by: Gabe Goodhart <redacted>
Signed-off-by: Sukriti-Sharma4 <redacted>
Co-authored-by: Gabe Goodhart <redacted>