How to get the tagset for hindi pos tagging?
I'm not familiar with NLTK but the tagset must come from the annotated corpus which was used to train the tagger. According to https://www.nltk.org/book/ch05.html (section 2.2), if you can find the name of the Hindi corpus you should be able to access the words with their tags with:

nltk.corpus.&lt;corpus&gt;.tagged_words()


Apparently this is the list of corpora available in NLTK: http://www.nltk.org/nltk_data/, it might help finding the one used to train the Hindi POS tagger.

Once you identify the corpus, it's likely that you can find explanations about the tagset and annotation process by searching for papers about its creation by the original authors.