Infer the right dtype for replay buffers.

[duburcqa]

Fix #65

[codecov-commenter]

Codecov Report

Merging #64 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #64   +/-   ##
=======================================
  Coverage   85.25%   85.25%           
=======================================
  Files          29       29           
  Lines        1634     1634           
=======================================
  Hits         1393     1393           
  Misses        241      241           

Flag	Coverage Δ
#unittests	85.25% <100.00%> (ø)

Impacted Files	Coverage Δ
tianshou/data/buffer.py	97.40% <100.00%> (ø)

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