Fixed Llama index bugs

[kshitiz-okahu]

Proposed changes

Fixed tow bugs related to llamaindex.

• Fixed bug where monocle will not instrument code langchain package is not installed.
• Fixed bug where token usage for llamaindex wrapper code would throw error. The code is modifieed to catch error and also updated to extract the tokens info correctly.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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