State the limitation when chunk / unbind meet a dynamic dim

[LeSingh1]

chunk and unbind both derive their number of outputs from the size of the dim they act on, so that dim has to be known at conversion time. Neither says so.

# torch.chunk(x, 3, dim=0) with a dynamic dim 0
TypeError: Cannot convert expression to float          # float(x.shape[dim])

# torch.unbind(x, dim=0) with a dynamic dim 0
TypeError: can't multiply sequence by non-int of type 'Symbol'   # [1] * x.shape[dim]

Neither message names the op or the limitation, and both come from inside the converter.

This raises a ValueError saying which dim is the problem and what to do instead, following what _adaptive_pool2d already does for the same situation. No model that converts today is affected — these paths only ever raised.

Also adds the first chunk tests; there were none.