Lazily trigger fused_norm for FusedAdam error on Windows

apex/optimizers/fp16_optimizer.py

@@ -2,17 +2,27 @@ import torch
 from torch._utils import _flatten_dense_tensors, _unflatten_dense_tensors
 import ctypes
 
-lib = ctypes.cdll.LoadLibrary(None)
-lib.THCudaHalfTensor_normall.argtypes=[ctypes.c_void_p, ctypes.c_void_p]
-lib.THCudaHalfTensor_normall.restype = ctypes.c_float
-
-def fused_norm(input):
-    if input.type() == 'torch.cuda.HalfTensor':
-        # 16384 is half 2 if you stare at it long enough
-        return lib.THCudaHalfTensor_normall(torch.cuda._state_cdata,
-            input._cdata, 16384)
-    else:
-        return input.norm()
+stashed_err = None
+try:
+    lib = ctypes.cdll.LoadLibrary(None)
+    lib.THCudaHalfTensor_normall.argtypes=[ctypes.c_void_p, ctypes.c_void_p]
+    lib.THCudaHalfTensor_normall.restype = ctypes.c_float
+
+    def fused_norm(input):
+        if input.type() == 'torch.cuda.HalfTensor':
+            # 16384 is half 2 if you stare at it long enough
+            return lib.THCudaHalfTensor_normall(torch.cuda._state_cdata,
+                input._cdata, 16384)
+        else:
+            return input.norm()
+except TypeError as err:
+    stashed_err = err
+    def fused_norm(input):
+        raise RuntimeError("Failed to create fused_norm. This may happen on Windows "
+              "because of lib = ctypes.cdll.LoadLibrary(None):  you can't "
+              "LoadLibrary with None.  Original exception message was ",
+              stashed_err)
+
 
 class FP16_Optimizer(object):
     """