fix acc topk's handling of the case when dim=0, fix tests as well (py…

…torch#64727)

Summary:
Pull Request resolved: pytorch#64727

the acc ops convertor for topk has a subtle bug (i found this while trying to introduce max/min)
the code does not differentiate between dim == None and dim ==0, but these are both different computations

Reviewed By: jfix71, 842974287

Differential Revision: D30833621

fbshipit-source-id: 6cd84e6ca4e95bb1a6d6465e61830b76808a9c78

torch/fx/experimental/fx2trt/converters/acc_ops_converters.py

@@ -813,7 +813,7 @@ def acc_ops_topk(network, target, args, kwargs, name):
 
     num_dims = len(input_val.shape) + (1 if network.has_implicit_batch_dimension else 0)
     k = kwargs["k"]
-    dim = (kwargs["dim"] if kwargs["dim"] else -1) % num_dims
+    dim = (kwargs["dim"] if kwargs["dim"] is not None else -1) % num_dims
     operation = trt.TopKOperation.MAX if kwargs["largest"] else trt.TopKOperation.MIN
     layer = network.add_topk(
         input_val, operation, k, get_axes_for_reduce_op(dim, network.has_implicit_batch_dimension)