Some layers have no gradient when backward.

[chenglong0313]

when I use DLA34, it seems that some layers have no gradient when backward.

Specifically， they are ：
module.encoder.base.level3.project.0.weight
module.encoder.base.level3.project.1.weight
module.encoder.base.level3.project.1.bias
module.encoder.base.level4.project.0.weight
module.encoder.base.level4.project.1.weight
module.encoder.base.level4.project.1.bias

i don't why, though maybe it doesn't affect training.