add missing annotations

[pmeier]

This was missed in #441

[codecov]

Codecov Report

Merging #443 into master will not change coverage.
The diff coverage is 100.0%.

@@          Coverage Diff           @@
##           master    #443   +/-   ##
======================================
  Coverage    90.7%   90.7%           
======================================
  Files          63      63           
  Lines        3091    3091           
======================================
  Hits         2806    2806           
  Misses        285     285           

Impacted Files	Coverage Δ
pystiche/enc/multi_layer_encoder.py	85.8% <100.0%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dd34c10...1fa5f03. Read the comment docs.