Patch SPE tokenizer not being available in older ASR Checkpoints
# Bugfix
- Older checkpoints might not have the `tokenizer.vocab` file when using SentencePiece tokenizers. This will cause model restoration to fail.
- Wrap in a try catch that catches ModelPT raised FileNotFoundError for this particular case to enable backward compatibility

Signed-off-by: smajumdar <titu1994@gmail.com>