update loss & a small change in requirements

[FFTYYY]

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[codecov-io]

Codecov Report

Merging #106 into master will increase coverage by 0.82%.
The diff coverage is 96.02%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage    77.5%   78.33%   +0.82%     
==========================================
  Files          74       75       +1     
  Lines        3717     3873     +156     
==========================================
+ Hits         2881     3034     +153     
- Misses        836      839       +3

Impacted Files	Coverage Δ
fastNLP/core/loss.py	90% <89.65%> (+17.27%)	⬆️
test/core/test_loss.py	99.15% <99.15%> (ø)
fastNLP/modules/encoder/variational_rnn.py	87.73% <0%> (-5.67%)	⬇️
fastNLP/fastnlp.py	92.19% <0%> (+1.41%)	⬆️
test/test_fastNLP.py	96.12% <0%> (+1.55%)	⬆️

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[xuyige]

the loss function names starts at line 99, loss.py should be more simpler
such as:
nll, cross_entropy, mse, ...