clarify torchao.float8 PyTorch version support (#1191)

We currently only support PyTorch version 2.5 and up.  

For historical context:
* we haven't supported 2.4 and below since the float8_experimental repository moved into torchao
* the error when we run the float8 README.md demo on PyTorch 2.4: https://gist.github.com/vkuzo/e8c06fd27429da0a6a079dcd46cc234f

In a future PR we can see what it would take to enable 2.4.1 support, but for now let's at least clarify the currently supported version.