fix imports and create datachain.torch

[dberenbaum]

Another attempt at fixing #52 and related issues around imports.

• Moves pillow to core dependencies since it is lightweight and need for basic operations like reading image files
• Renames rest of cv optional deps to torch since they are all torch-based and their usage is broader than cv
• Moves all torch code to datachain.torch to simplify imports and unify torch-related code

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[skshetry]

@dberenbaum, can we make these imports lazy instead?

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[dberenbaum]

@skshetry Do you mean putting the imports inside each individual function/method? IMHO it's not worth the added complexity, especially since this is already extracted out to datachain.torch, but would be curious to hear from others.

Edit: The only place where any torch functionality is called in the core code is DataChain.to_pytorch(), where it is imported lazily.