Fix a couple of undefined name in the error string (#9221)
Fix a couple of undefined name in the error string (#9221)

This fix tries to fix a couple of issues in the error
string of `def switch(..` where `cond_ndim` and `expr_ndim`
are undefined, and should be replaced with `ndim_cond` and `ndim_expr`.

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>
keras-team/keras,keras-team/keras