Allocate all GPU memory on startup

Suggested by @slush0 on Discord.

It would be great to allocate all GPU memory on startup (including max space for attention cache and max space for intermediate tensors created during backward).

Right now, the memory is allocated on-demand, and people may see OOMs only after a few days of running the server. Allocating everything right away would allows to see OOMs right away and choose a correct number of blocks before leaving a server to run in the background.

Any other way to check if we have enough GPU memory on startup would be also good.