[tosa] : Handle CHW input for avgpool2d.

[sahas3]

This PR fixes two issues:

1. Avg(Max)Pool2d allows (C, H, W) as input which causes a failure when creating tosa.avg_pool2d or tosa.max_pool2d as those ops expects 4D (N, H, W, C) tensor. Fix is to add a N=1 dimension before creating the tosa ops.
2. Avg(Max)Pool2d also allows kernel/stride to be specified as a tuple of single int, in which case the value is repeated for both H and W dims. This is currently not handled as well causing a segv when trying to access kernel[1]/stride[1]. Fix is to expand kernel/stride to be size 2 by repeating the first element.

[sjarus]

LGTM, thanks for this!

[sjarus]

Could you please change the subject to [tosa] and use that as the prefix instead of '[Task]' @sahas3 ?