Einsum (#1269)

* einsum initial

* fix comma break

* sum axis was wrong

* small cleanups

* python binding

* changed bindings to resemble numpy

* remove todo comment

* comment changes

* add count of operands/inputs

* fail fast if operands list is empty

* ignore comma if no output

* einsum path matching numpy

* getting somewhere with path

* remove print

* it passes the first test

* moved einsum tests to seperate file

* seperated einsum path

* moved einsum naive

* remove space from equation

* fast fail if no operands passed

* update tests and remove printf

* small cleanup

* some more cleanups

* removed python helper file

* ack

* utilize std for finding min in vector

* duplicate def

* remove the tuple as it was unreadable

* moved einsum_naive back to ops

* remaining isn't needed

* avoid creating another set

* cleanup

* greedy path, start of naive einsum

* more einsum

* fix some bugs

* some more fixes, tests pass

* benchmark

* some simplify

* fix einsum and test

Co-authored-by: Angelos Katharopoulos <a_katharopoulos@apple.com>

* add a bunch more tests and fix a bunch more bugs

* some docs nits

---------

Co-authored-by: dc-dc-dc <dgcruz983@gmail.com>
Co-authored-by: Angelos Katharopoulos <a_katharopoulos@apple.com>

tests/CMakeLists.txt

@@ -26,6 +26,7 @@ target_sources(tests PRIVATE
   custom_vjp_tests.cpp
   creations_tests.cpp
   device_tests.cpp
+  einsum_tests.cpp
   eval_tests.cpp
   fft_tests.cpp
   load_tests.cpp