Enable lazy tensor allocation

- The previous ref_model was allocating the memory for all tensors in
the graph upfront which is unnecessary and wasteful.
- This patch changes to only allocate initial input tensors on the entry
point using the allocateInputTensor() function.
- The output tensors are ensured to have been allocated before executing
a node. The output tenosrs are the inputs for the next node.
- When a node's evaluation is finished, its input tensors will be freed
if they will no longer be used by anyone else.

Signed-off-by: Jerry Ge <jerry.ge@arm.com>
Change-Id: Ibb3e8c9e6344f6cd9eb20532a03b2097b93247f9