test_dnn failing

[QiQi-Guo]

After build dlib and tests, I'm getting this error when running ./dtest --runall

Running test_dnn /

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!! TEST FAILED: test_dnn !!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Failure message from test:

Error occurred at line 2011.
Error occurred in file /home/gq/dlib/dlib/test/dnn.cpp.
Failing expression was res.
Gradient error in parameter #0. Relative error: 0.848535
expected derivative: -3.19285
output derivative: -0.483604
iteration: 0

Testing Finished
Total number of individual testing statements executed: 470123422
Number of failed tests: 1
Number of passed tests: 143

[arrufat]

It's a known issue in the layer_norm gradients: #2902