Clarify that the `cog` Python library is a library, apart from the cog CLI

[zeke]

I've heard from a couple users that they tried to pip install cog expecting to end up with a cog CLI, but of course that didn't work. We should update our docs to clarify what's going on here.

Here's why:

cog CLI

The cog CLI is written in Golang and is distributed via binaries GitHub Releases (and Homebrew for macOS users):

cog build ...
cog predict ...
cog push ...

cog Python package

The cog package on PyPi is a different thing. It's the Python library you import into your model code when writing a Predictor:

from cog import BasePredictor, Path, Input

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Related issues:

• Install Python library from PyPi #147