[utils]

Utils for tensorflow backend support code.

GPU/CPU backing device management utils

• move_layer_to_device: Create a copy of an input keras layer placed on a given device.
• preserve_tensor_device: Wrap a tensor-processing function to have it run on its inputs' device.
• select_device: Select a backing device to use based on inputs and availability.

Loss function management utils

• build_keras_loss: Type-check, deserialize and/or wrap a keras loss into a Loss object.

Better support for sparse tensor structures

• add_indexed_slices_support: Run a function on a pair of tensors, adding support for IndexedSlices.