Batch not serializable

[duburcqa]

• I have marked all applicable categories:
  • exception-raising bug
  • RL algorithm bug
  • documentation request (i.e. "X is missing from the documentation.")
  • new feature request
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Batch instances are creating infinite recursive loop when used in argument of a multiprocessing Pipe. Here is a snippet to reproduce the issue.

from tianshou.data import Batch
from multiprocessing import Pipe
(p, c) = Pipe()
c.send(Batch(a=1.0))
print(p.recv())

Similarly, using Pickle without multiprocessing produces the same issue.

from tianshou.data import Batch
import pickle
pickle.dump(Batch(a=1.0), open("save.p", "wb"))                                                                                                                                                     
pickle.load(open("save.p", "rb"))

I'm going to open a PR that fixes the issue.