add support for CPU and for MPS. I had to change a few cosmetic things. I also discovered I think a bit of a bug, where I was casting wte to bfloat16 in the wrong place (the model init) instead of in init_weights

nanochat/loss_eval.py

@@ -33,7 +33,7 @@ def evaluate_bpb(model, batches, steps, token_bytes):
         loss2d = model(x, y, loss_reduction='none') # (B, T)
         loss2d = loss2d.view(-1) # flatten
         y = y.view(-1) # flatten
-        if (y < 0).any():
+        if (y.int() < 0).any(): # mps does not currently have kernel for < 0 for int64, only int32
             # slightly more complex code path if some target tokens are ignore_index (e.g. -1)
             # any target token < 0 is to be ignored: do NOT index token_bytes with negatives
             valid = y >= 0