Fix bug of empty input in gelu.cu

[iProzd]

#675 and #742 maybe the same bugs when:

1. using gelu as activation function (GPU environment);
2. there's one type (or types) of atom not showing in the system but in the type_map;
   this causes the empty input in gelu.cu and thus breaks.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.96%. Comparing base (b145ca3) to head (a958bcb).

Additional details and impacted files

@@           Coverage Diff           @@
##            devel     #800   +/-   ##
=======================================
  Coverage   73.96%   73.96%           
=======================================
  Files          84       84           
  Lines        6588     6588           
=======================================
  Hits         4873     4873           
  Misses       1715     1715           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[njzjz]

Is our gelu much faster than TensorFlow's (tensorflow/tensorflow#41178)? It seems that they implement gelu only using some python codes. See https://github.com/tensorflow/tensorflow/blob/v2.5.0/tensorflow/python/ops/nn_ops.py#L3505-L3548

[amcadmus]

Is our gelu much faster than TensorFlow's (tensorflow/tensorflow#41178)? It seems that they implement gelu only using some python codes. See https://github.com/tensorflow/tensorflow/blob/v2.5.0/tensorflow/python/ops/nn_ops.py#L3505-L3548

Yes, that's why we implement gelu by ourselves