add test and update error message for functional.py::load_sp_model

[astaff]

This PR:

• adds test for unsupported input type test_sentencepiece_unsupported_input_type (review comment)
• updates error message listing valid input types and type of input given. (review comment)

Notes:

• Should we raise TypeError instead of RuntimeError?
• Type annotation and static type checking is another option to do that.

[zhangguanheng66]

LGTM. Please sync with the master branch so the failed CI tests will go away.

[zhangguanheng66]

I like TypeError.

[codecov]

Codecov Report

Merging #984 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #984      +/-   ##
==========================================
+ Coverage   77.67%   77.70%   +0.03%     
==========================================
  Files          44       44              
  Lines        3099     3099              
==========================================
+ Hits         2407     2408       +1     
+ Misses        692      691       -1     

Impacted Files	Coverage Δ
torchtext/data/functional.py	89.47% <100.00%> (+2.63%)	⬆️

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