Vela: Fix diff in mean op

  - Extend ifm/ofm dimensions explicitly in mean op
    This fix a bug when ifm/ofm shape has different dimensions
    e.g. IFM=1x19x18x25 axis=2 OFM=1x19x25,
         the ofm_shape should be 1x19x1x25, not 1x1x19x25
  - Fix wrong weight shape

Change-Id: I269eb71ea56c09deee2aa6c6433d9b2baa98a113
Signed-off-by: Diqing Zhong <diqing.zhong@arm.com>