message: [Model] don't create part_grad for input tensor
diff: @@ -979,6 +979,7 @@ void FFModel::create_data_parallel_partition_with_diff_dims(const Tensor& tensor
  assert(tensor.numDim == NDIM);
  if (config.computationMode == COMP_MODE_TRAINING) {
  // Current assume forward and grad share the same index space
  + if (tensor.region_grad != LogicalRegion::NO_REGION)
  assert(tensor.region.get_index_space() == tensor.region_grad.get_index_space());
  }
  Context ctx = config.lg_ctx;
  @@ -1007,6 +1008,7 @@ void FFModel::create_data_parallel_partition_with_diff_dims(const Tensor& tensor
  assert(runtime->is_index_partition_complete(ctx, ip));
  part_fwd = runtime->get_logical_partition(ctx, tensor.region, ip);
  if (config.computationMode == COMP_MODE_TRAINING) {
  + if (tensor.region_grad != LogicalRegion::NO_REGION)
  part_bwd = runtime->get_logical_partition(ctx, tensor.region_grad, ip);
  } else {
  part_bwd = LogicalPartition::NO_PART;
  