FSDP: better traceback for dtype assertion (#912)

fairscale/nn/misc/flatten_params_wrapper.py

@@ -279,8 +279,14 @@ class FlattenParamsWrapper(nn.Module):
         shared_param_memo: Dict[nn.Parameter, Tuple[str, nn.Module, str]] = {}
         shared_param_infos = []
         params = []
+        fp32 = []
+        fp16 = []
         for module_name, m in self.named_modules():
             for n, p in m.named_parameters(recurse=False):
+                if p.dtype != torch.float16:
+                    fp32.append(module_name)
+                else:
+                    fp16.append(module_name)
                 if p is not None and (m, n) in p_set:
                     if p in shared_param_memo:
                         mname, shared_m, shared_n = shared_param_memo[p]
@@ -290,8 +296,10 @@ class FlattenParamsWrapper(nn.Module):
                         param_infos.append((module_name, m, n))
                         params.append(p)
         del shared_param_memo
-
-        assert len(set(p.dtype for p in params)) == 1, "expects all parameters to have same dtype"
+        fp16_msg, fp32_msg = ",".join(fp16), ",".join(fp32)
+        assert (
+            len(set(p.dtype for p in params)) == 1
+        ), f"expects all parameters to have same dtype: fp32: {fp32_msg} \n fp16: {fp16_msg} "
         assert len(set(p.requires_grad for p in params)) == 1, "expects all parameters to have same requires_grad"
         assert len(params) == len(set(params)), "params list should not have dups"
         return params, param_infos, shared_param_infos