Remove support for adjoints of nodes

Python capsules support for ngraph::Function

* Pad: add pad mode SYMMETRIC

* Style.

* Disable UT on unsupported backends.

* Make required numpy version explicit.

* Read setup requirements from requirements.txt

Restore DEFAULT_FLOAT_TOLERANCE_BITS logic in lrn, sigmoid, softmax. Should allow re-enabling PlaidML tests (#3277)

* Added rpath setting for PlaidML backend

* Fix colons

* Separate build and install rpath

* Explicitly set target properties

Cyphers/s barannikov

* Update documentation for utility test functions.

* Remove unnecesary code and update comment.

* Added elu operator to Python API.

* Added missing file.

* Specified elu function description.

* Expand docstring

* [Py] Added test with scalar for elu operator.

* Bugfix

*  [Py] Changed input type in elu test.

* Update test_ops_binary.py

* [Py] Syntax bugfix.

* [Py] Added elu operator to list in documentation.

* Qualify `is_commutative` method with `const`
* Override it for `Equal`, `Minumum`, `NotEqual` ops
* Ensure that all overrides of the method are `public`

Other operations which have already been converted to new op form accept
`Output<Node>`` by const reference. `BatchNorm*` ops were an exception.

* Fix perf regression in vgg16

* Make switch generic

* Remove unused variables

* Review comments

* Remove unused function parameters

* trivial commit to restart CI

* Split backend_test.in.cpp

* style

* Move backend_{graph_comparison,mlir}.in.cpp back

* Move backend_mlir.in.cpp into backend/ dir

* Disable some tests in PlaidML manifest