gradient value is very small in energy constraints situation

I tried to implement the knowledge network in the energy constraints situation on my own task. However, I found that the `grad` from the `get_mean_shift` function is very small, which will not have any effect on the original mean. For example, the `grad` values are below:

```
tensor([[[[ 2.7554e-08]],

[[-8.8887e-09]],

[[ 8.4224e-09]],

...,

[[ 5.1067e-12]],

[[ 3.8011e-11]],

[[-2.2646e-12]]],

[[[-5.8177e-09]],

[[ 1.4201e-09]],

[[-2.9926e-10]],

...,

[[ 7.6124e-12]],
...
[[-4.7238e-11]],

[[-5.8617e-11]],

[[ 4.8769e-11]]]], device='cuda:0')
```
So what is your case?