bugfix: if no kernel found for kernel type, paddle will crash directly now

[jacquesqiao]

currently, paddle will find a proper kernel for a kernel key and then run this kernel:

  auto kernel_iter = kernels.find(expected_kernel_key);
  kernel_iter->second->Compute(
      ExecutionContext(*this, new_scope, *new_dev_ctx));

but kernels.find(expected_kernel_key) may not find a kernel for expected_kernel_key, then kernel_iter->second->Compute will crash directly without any information, we should add a check to kernel_iter.