[New] Add MTL16Loader

[lyhuang18]

added MTL16Loader for text classification

[codecov-io]

Codecov Report

Merging #168 into dev0.5.0 will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           dev0.5.0     #168      +/-   ##
============================================
- Coverage     64.68%   64.63%   -0.05%     
============================================
  Files            88       88              
  Lines          9083     9083              
============================================
- Hits           5875     5871       -4     
- Misses         3208     3212       +4

Impacted Files	Coverage Δ
fastNLP/core/callback.py	66.46% <0%> (-1.2%)	⬇️

---

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 1a4c3c2...c78811f. Read the comment docs.