[batchnorm] Optimize batch norm backwarding

[batchnorm] Optimize batch norm backwarding

Remove the extra full size extra memory requirement as the cost of the
reduced memory. The difference in memory requirmement can be
significant. Earlier memory requirement was b*c*h*w vs now it is just c
where the assumption is that batch norm if is normalizing along
axis=channel.
This is achieved by reordering of the operations.
Note: this change has no performance impact.

Signed-off-by: Parichay Kapoor <pk.kapoor@samsung.com>