- Remove PyArrayObject pointers from CTC context
- Remove activations copy by assuming activations array is C-contiguous
- Add missing CTC context deallocations when errors occur
- Move allocation of CTC context from heap back to the stack for
  automatic deallocation
- Add separate function name for when gradient computations are
  disabled, i.e. ctc_cost_cpu_no_grad
Signed-off-by: João Victor Tozatti Risso <joaovictor.risso@gmail.com>