* Rename PlaidML_Executable::save -> save_as_format

* Repair regression in PlaidML tensor impl

This was caused by the recent removal of the offset parameter for tensor read/write operations -- we missed a
spot where read/write were being called for synchronization purposes.

* Disable a few more PlaidML tests pending triage

* Skip elision of reshape->reshape

It turns out this doesn't work, because the downstream reshape's input_order axis vector is incorrect if the
upstream reshape is removed.

* Add element type to PlaidML tensor debug output

* Use nGraph booleans for PlaidML boolean data

We'd previously been using i8; that's been deprecated for boolean data now that we have an explicit boolean
element type.

* Set PlaidML convolution output shapes correctly

We weren't transposing the output shape; we were computing the right data, but the incorrect shape metadata
causes validation to fail.

* Add a PlaidML implicit broadcast op

Better nGraph shape validation was tripping up PlaidML's use of a reshape to replace explicit broadcasts with
implicit NumPy-style broadcasts (since the reshape's output shape would be incorrect for the downstream
elementwise operation).  Adding this implicit broadcast operation lets PlaidML tell nGraph something useful
about the shapes, making validation pass (when it's otherwise correct).