Enhance support for 'tf.IndexedSlices' in 'TensorflowVector'.

* Implement a public util to wrap tensorflow operations in order to preserve
  `tf.IndexedSlices` structures and run appropriate computations with them.
* Deploy the former wrapper to cover all usual operations in the backend of
  `TensorflowVector`.
* Refactor the use of the device-placement-handling wrapper in the backend of
  `TensorflowVector` to reduce runtime overheads and factor it with the new
  indexed-slices-handling wrapper.