[TOPI][Relay] Fix default `out_dtype` for `conv2d_NCHWc` and Relay (#2707)

topi/python/topi/x86/conv2d.py

@@ -294,6 +294,7 @@ def _alter_conv2d_layout(attrs, inputs, tinfo, F):
     padding = attrs.get_int_tuple("padding")
     strides = attrs.get_int_tuple("strides")
     dilation = attrs.get_int_tuple("dilation")
+    out_dtype = attrs["out_dtype"]
 
     layout_name = 'layout' if F == sym else 'data_layout'
 
@@ -301,7 +302,7 @@ def _alter_conv2d_layout(attrs, inputs, tinfo, F):
     kh, kw = attrs.get_int_tuple("kernel_size")
 
     dtype = data.dtype
-    out_dtype = dtype if attrs["out_dtype"] == "same" else attrs["out_dtype"]
+    out_dtype = dtype if out_dtype in ("same", "") else out_dtype
     is_depthwise = groups == in_channel and groups == out_channel
 
     # only optimize for NCHW