chore: bump pytorch to >= 2.7

[lianakoleva]

Addresses #2122 - B200s will be supported out of the box.

[t-vi]

Thank you @lianakoleva

[Borda]

Why is it needed? 🤔
since it installs 2.8 anyway
and this means no one can run it with 2.6 anymore...

[lianakoleva]

i think there are different defaults that will be installed depending on user OS? if we require 2.7 then 2.7 will be installed

[Borda]

So bump it if you need it for the target env, don't force everyone else...
pip install litgpt torch>=2.7

[lianakoleva]

I don't know of a straightforward way to detect B200 in pyproject.toml, do you have an idea of how to make it work out of the box or were you thinking including it as an extra (e.g., pip install litgpt[b200]) and adding documentation?