class_docstr: Why `MutableTensorInfo`?
  
  We need a cluster to transfer meta information of mutable tensor, between
  server and client, as over the HTTP web session.
  
  Thus we design an internal-only `MutableTensorInfo` type as a container
  for those information.
  
  A `MutableTensor` can be initialized from
  
      - a info, which contains the metadata
      - a `mutable_api`, which will be used to request the backend API
      - a `loop`, which will be used to execute `__setitem__` (and `__getitem__`)
        synchronously to make the API more user-friendly.
code: def default_value(self):
      return self._default_value
  
file_path: mars/mars/services/mutable/core.py
imports: import asyncio
label_desc: Use image node_id 5 for calling the MutableTensorInfo obj's underlying member method code with example usage: obj.default_value() and returns: self