Add the ability to specify a specific date with the nightly channel

[pmeier]

When debugging it is often useful to install nightlies from a specific day. Right now, we need to do

$ ltt install --pytorch-channel=nightly torch==0.12.0.dev20220510 torchvision==0.12.0.dev20220510

This is inconvenient since

1. we need to repeat the date if we want to install multiple PyTorch distributions
2. we need to know the exact version of the PyTorch distribution at the given date

Maybe we can have something like

$ ltt install --pytorch-channel=nightly:20220510 torch torchvision