[Relay][Frontend][Bugfix] Fix bug in converting slice_axis when axis is negative (apache#2739)

* bug fix

* trigger ci

Author: icemelon

python/tvm/relay/frontend/mxnet.py

@@ -200,6 +200,9 @@ def _mx_slice_axis(inputs, attrs):
     axis = attrs.get_int("axis")
     ax_beg = attrs.get_int("begin")
     ax_end = attrs.get_str("end")
+    if axis < 0:
+        axis += len(shape)
+    assert axis >= 0 and axis < len(shape)
     if ax_end == "None":
         ax_end = int(shape[axis])
     else:

tests/python/frontend/mxnet/test_forward.py

@@ -352,6 +352,7 @@ def verify(shape, axis, begin, end):
     verify((3, 4), 0, 1, None)
     verify((3, 4), 1, 0, 2)
     verify((3, 4), 1, -3, -1)
+    verify((3, 4), -1, -3, -1)
 
 
 if __name__ == '__main__':
@@ -380,4 +381,4 @@ def verify(shape, axis, begin, end):
     test_forward_broadcast_ops()
     test_forward_elemwise_ops()
     test_forward_scalar_ops()
-    test_forward_slice_axis()
+    test_forward_slice_axis()