[Bug] [Relay][MXNet] wrong conversion logic in Repeat operator about axis

[jikechao]

Bug description

According to the MXNet documentation, the default value of axis is None rather than 0. They are different.

None: uses the flattened input array and return a flat output array.
0: repeats the data along the first axis.

---

For example:

x = [[ 1, 2], [ 3, 4]]
repeat(x, repeats=2) = [ 1., 1., 2., 2., 3., 3., 4., 4.]
repeat(x, repeats=2, axis=0) = [[ 1., 2.], [ 1., 2.], [ 3., 4.], [ 3., 4.]]

cc @shingjan

[jikechao]

This bug has been fixed by #15891