Added pickle support for lightning.gpu device's DevPool attribute (#772)

**Context:**
Allowing the `qml.device("lightning.gpu", wires=...)._dp` argument,
which is a python `DevPools()` object to be pickle-able through the C++
layer.

**Description of the Change:**
- Updated Devpools object to have a `refresh()` method
- Updated TSQueue to have a clear method
- Add bindings for pickle to DevPool

---------

Co-authored-by: ringo-but-quantum <[email protected]>
Co-authored-by: Ali Asadi <[email protected]>

Author: 3 people

.github/CHANGELOG.md

@@ -159,7 +159,7 @@
 
 This release contains contributions from (in alphabetical order):
 
-Ali Asadi, Amintor Dusko, Lillian Frederiksen, Pietropaolo Frisoni, David Ittah, Vincent Michaud-Rioux, Lee James O'Riordan, Mudit Pandey, Shuli Shu
+Ali Asadi, Amintor Dusko, Lillian Frederiksen, Pietropaolo Frisoni, David Ittah, Vincent Michaud-Rioux, Lee James O'Riordan, Mudit Pandey, Shuli Shu, Jay Soni
 
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