Send list as argument to schedule_conv2d (apache#4358)

When getting cuda schedule passing single tensor seem to work but after changing target to "llvm" causes assert.
Sending list on other hand makes both cuda and llvm targets happy.
See https://discuss.tvm.ai/t/solved-simple-example-error-attributeerror-tensorslice-object-has-no-attribute-op/2245/3

tutorials/topi/intro_topi.py

@@ -135,7 +135,7 @@
 with tvm.target.create("cuda"):
     conv = topi.nn.conv2d(data, kernel, strides=1, padding=2, dilation=1)
     out = topi.nn.relu(conv)
-    sconv = topi.generic.nn.schedule_conv2d_nchw(out)
+    sconv = topi.generic.nn.schedule_conv2d_nchw([out])
     print(tvm.lower(sconv, [data, kernel], simple_mode=True))
 
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