[BugFix] make TensorDictReplayBuffer.extend call super().extend with stacked_td

[vmoens]

Description

Calls ReplayBuffer.extend with stacked tensordicts rather than stacked tensordicts or a list of tensordicts depending on the input.
cc @adityagoel4512

[codecov]

Codecov Report

Merging #634 (08f4454) into main (dd9a98f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #634   +/-   ##
=======================================
  Coverage   87.92%   87.92%           
=======================================
  Files         125      125           
  Lines       24218    24218           
=======================================
  Hits        21293    21293           
  Misses       2925     2925           

Flag	Coverage Δ
habitat-gpu	29.17% <ø> (ø)
linux-cpu	85.10% <100.00%> (ø)
linux-gpu	86.42% <100.00%> (ø)
linux-outdeps-gpu	75.65% <100.00%> (ø)
linux-stable-cpu	84.98% <100.00%> (ø)
linux-stable-gpu	86.30% <100.00%> (ø)
macos-cpu	84.88% <100.00%> (ø)
olddeps-gpu	76.51% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
torchrl/data/replay_buffers/rb_prototype.py	86.40% <100.00%> (ø)

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