Issue:
Once change LOG_BLOCK_IN or LOG_BLOCK_OUT into > 4 value, when run vta
“Simple Matrix Multiply” or load vta, vta would crash at vta_conv2d.py.

Analysis:
This issue caused by resnet18 logic of vta_conv2d.py which have
in_filter minmum size that is 16. > 4 value would cause such in_filter
check failed then make xfer_size be empty and find_schedules function
return a empty list finally cause crash.

Solution:
add the empty list check.