[TOPI] Fix tuple unpack in conv2d NCHWc int8 (#13761)

Fixes a tuple being incorrectly unpacked (too many elements expected) in a Conv2D TOPI int8 op, similarly to #13566 which addressed the issue specifically in the ARM implementation (which calls the function modified in this PR). 

This explicit unpacking helps improve flexibility with other incoming layouts.

python/tvm/topi/nn/conv2d.py

@@ -515,9 +515,9 @@ def conv2d_NCHWc_int8(
 
     n, ic_chunk, ih, iw, ic_bn = get_const_tuple(data.shape)
     in_channel = ic_chunk * ic_bn
-    oc_chunk, ic_chunk_group, kernel_height, kernel_width, _, oc_bn, _ = get_const_tuple(
-        kernel.shape
-    )
+    oc_chunk, ic_chunk_group, kernel_height, kernel_width, _, oc_bn = get_const_tuple(kernel.shape)[
+        :6
+    ]
     groups = ic_chunk // ic_chunk_group
 
     dilated_kernel_h = (kernel_height - 1) * dilation_h + 1