Update tensors and autograd (#1185)

* Update tensor_tutorial.py

* Update autograd_tutorial.py

* Fix import

* Fix import

* Fix import

* Fixes issues in tensor, updates autograd

* Adds "what is pytorch" to homepage

* Fixes formatting

* Fix typo

* Fixes typo

* fix failing test

* Make suggested changes

Adds a DAG figure for autograd, fixes formatting issues, improve readability

Co-authored-by: Brian Johnson <brianjo@fb.com>

Author: subramen