some kind of warning or something when this is first executed to make people...

some kind of warning or something when this is first executed to make people aware that a cast happens and the operation quantization is performed in fp16.

bitsandbytes/autograd/_functions.py

 import operator
+import warnings
+
 import torch
 import bitsandbytes.functional as F
 
@@ -229,6 +231,8 @@ class MatMul8bitLt(torch.autograd.Function):
 
         # Cast A to fp16
         A_dtype = A.dtype
+        if A_dtype != torch.float16:
+            warnings.warn(f"MatMul8bitLt: temporarily casting input matrix from {A_dtype} to float16")
         A = A.to(torch.float16)
 
         # 1. Quantize A