Add support for creating docsets. (pytorch#1276)

Docsets are an offline documentation format introduced by Dash.app and
supported by Zeal and some other open-source clones.

docs/Makefile

@@ -12,7 +12,14 @@ BUILDDIR      = build
 help:
 	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
 
-.PHONY: help Makefile
+docset: html
+	doc2dash --name $(SPHINXPROJ) --icon $(SOURCEDIR)/_static/img/pytorch-logo-flame.png --enable-js --online-redirect-url http://pytorch.org/docs/ --force $(BUILDDIR)/html/
+
+	# Manually fix because Zeal doesn't deal well with `icon.png`-only at 2x resolution.
+	cp $(SPHINXPROJ).docset/icon.png $(SPHINXPROJ).docset/[email protected]
+	convert $(SPHINXPROJ).docset/[email protected] -resize 16x16 $(SPHINXPROJ).docset/icon.png
+
+.PHONY: help Makefile docset
 
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).