Refactored `input` (#8202)
Refactored `input.py`. The same as
https://github.com/langchain-ai/langchain/pull/7961 #8098 #8099
input.py is in the root code folder. This creates the `langchain.input:
Input` group on the API Reference navigation ToC, on the same level as
Chains and Agents which is incorrect.
Refactoring:
- copied input.py file into utils/input.py
- I added the backwards compatibility ref in the original input.py.
- changed several imports to a new ref
@hwchase17, @baskaryan
