fixed bug when changing default activation function in wavenet #295

[tdenimal]

*Issue #295

Description of changes:

Changed act_type by activation to be compliant with mxnet.gluon.nn.Activation syntax , see http://mxnet.incubator.apache.org/versions/1.4.1/api/python/gluon/nn.html#mxnet.gluon.nn.Activation

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[szha]

Job PR-299/1 is complete.
Docs are uploaded to http://gluon-ts-staging.s3-accelerate.dualstack.amazonaws.com/PR-299/1/index.html

[codecov-io]

Codecov Report

Merging #299 into master will increase coverage by 3.81%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #299      +/-   ##
==========================================
+ Coverage   39.62%   43.43%   +3.81%     
==========================================
  Files         142      142              
  Lines        7940     7940              
==========================================
+ Hits         3146     3449     +303     
+ Misses       4794     4491     -303

Impacted Files	Coverage Δ
src/gluonts/model/wavenet/_network.py	14.2% <ø> (ø)	⬆️
src/gluonts/support/util.py	41.59% <0%> (+0.88%)	⬆️
src/gluonts/core/serde.py	57.72% <0%> (+1.62%)	⬆️
src/gluonts/model/forecast.py	54.31% <0%> (+3.04%)	⬆️
src/gluonts/model/r_forecast/_predictor.py	19.51% <0%> (+19.51%)	⬆️
src/gluonts/time_feature/holiday.py	90% <0%> (+23.33%)	⬆️
src/gluonts/model/deepstate/_network.py	26.38% <0%> (+26.38%)	⬆️
src/gluonts/model/deepstate/issm.py	33.05% <0%> (+33.05%)	⬆️
src/gluonts/model/deep_factor/_network.py	37.5% <0%> (+37.5%)	⬆️
src/gluonts/model/deepstate/_estimator.py	37.93% <0%> (+37.93%)	⬆️
... and 11 more

[lostella]

Looks good, thank you!