Feat bedrock cohere support

[massi-ang]

Description:
Added support for Cohere command model via Bedrock.
With this change it is now possible to use the cohere.command-text-v14 model via Bedrock API.

About Streaming: Cohere model outputs 2 additional chunks at the end of the text being generated via streaming: a chunk containing the text <EOS_TOKEN>, and a chunk indicating the end of the stream. In this implementation I chose to ignore both chunks. An alternative solution could be to replace <EOS_TOKEN> with \n

Tests: manually tested that the new model work with both llm.generate() and llm.stream().
Tested with temperature, p and stop parameters.

Issue: #11181

Dependencies: No new dependencies

Tag maintainer: @baskaryan

Twitter handle: mangelino

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Name	Status	Preview	Comments	Updated (UTC)
langchain	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 4, 2023 0:47am

[hwchase17]

looks solid to me!

[demchuk-alex]

Thanks a lot, @massi-ang I was going to add this fix as well but found that somebody did it already. I've got a project that depends on Cohere on a Bedrock so I'd much appreciate it if it's merged as soon as possible :) I'm using a crutch of inheritance and hacking a few methods :) Thanks.