Add copy button and automatic cross-refs to external projects in docs (#3868)

docs/requirements.txt

 sphinx==2.4.4
 sphinx-gallery>=0.9.0
+sphinx-copybutton>=0.3.1
 matplotlib
 numpy
 -e git+git://github.com/pytorch/pytorch_sphinx_theme.git#egg=pytorch_sphinx_theme

docs/source/conf.py

@@ -21,10 +21,6 @@
 # import sys
 # sys.path.insert(0, os.path.abspath('.'))
 
-from pathlib import Path
-import os
-
-import torch
 import torchvision
 import pytorch_sphinx_theme
 
@@ -48,7 +44,9 @@ extensions = [
     'sphinx.ext.mathjax',
     'sphinx.ext.napoleon',
     'sphinx.ext.viewcode',
+    'sphinx.ext.duration',
     'sphinx_gallery.gen_gallery',
+    "sphinx_copybutton"
 ]
 
 sphinx_gallery_conf = {
@@ -202,7 +200,10 @@ texinfo_documents = [
 # Example configuration for intersphinx: refer to the Python standard library.
 intersphinx_mapping = {
     'python': ('https://docs.python.org/', None),
+    'torch': ('https://pytorch.org/docs/stable/', None),
     'numpy': ('http://docs.scipy.org/doc/numpy/', None),
+    'PIL': ('https://pillow.readthedocs.io/en/stable/', None),
+    'matplotlib': ('https://matplotlib.org/stable/', None),
 }
 
 # -- A patch that prevents Sphinx from cross-referencing ivar tags -------