docstr: _shard_as: shard the tensor w.r.t a distributed specification.
  Assuming the tensor passed in is a global (replicated) tensor.
  Args:
      tensor (torch.Tensor): a global (replicated) tensor before shard
      dist_spec (_DistSpec): the distributed spec. to be sharded as.
      pg (ProcessGroup): the process group of the corresponding colotensor
  Returns:
      torch.Tensor: a torch tensor after sharded.
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code: def _shard_as(
      tensor: torch.Tensor,
      old_dist_spec: _DistSpec,
      dist_spec: _DistSpec,
      pg: ProcessGroup,
  ) -> torch.Tensor:
      """_shard_as: shard the tensor w.r.t a distributed specification.
      Assuming the tensor passed in is a global (replicated) tensor.
      Args:
          tensor (torch.Tensor): a global (replicated) tensor before shard
          dist_spec (_DistSpec): the distributed spec. to be sharded as.
          pg (ProcessGroup): the process group of the corresponding colotensor
      Returns:
          torch.Tensor: a torch tensor after sharded.
      """
      assert (
          old_dist_spec.placement.value == "r"
      ), f"The old_dist_spec of DistSpecManager._shard_as must be REPLICATE!"
      DistSpecManager._sanity_check(old_dist_spec, dist_spec)
  
      chunk = tensor
      idx = pg.tp_local_rank()
      num_parts = prod(dist_spec.num_partitions)
      for i, dim in enumerate(dist_spec.dims):
          num_parts //= dist_spec.num_partitions[i]
  
          chunk_size = divide(
              tensor.size(dim), dist_spec.num_partitions[i]
          )
          chunk = chunk.narrow(
              dim, idx // num_parts * chunk_size, chunk_size
          )
          idx %= num_parts
      return chunk.clone().detach().contiguous()
  
imports: from contextlib import contextmanager
  import torch
  import torch.distributed
  from numpy import prod
  from colossalai.legacy.tensor.distspec import DistPlacementPattern, _DistSpec
  from colossalai.legacy.tensor.process_group import ProcessGroup
label_desc: Use image node_id 2 for calling the DistSpecManager obj's underlying member method code with example usage: obj._shard_as(tensor, old_dist_spec, dist_spec, pg) and returns: chunk