Rename expt.tensornet device to default.tensor

[josh146]

Context: The experimental TensorNetwork simulator had a short name that did not fit the PennyLane convention for device shortnames.

Description of the Change:

• expt.tensornet has become default.tensor

• expt.tensornet.tf has become default.tensor.tf

Benefits:

• The expt prefix has been replaced by default, to bring it inline with existing PennyLane simulators.

• tensornet has been shortened to tensor

Possible Drawbacks: n/a

Related GitHub Issues: n/a

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@2096a60). Click here to learn what that means.
The diff coverage is 75%.

@@            Coverage Diff            @@
##             master     #495   +/-   ##
=========================================
  Coverage          ?   99.05%           
=========================================
  Files             ?       73           
  Lines             ?     4457           
  Branches          ?        0           
=========================================
  Hits              ?     4415           
  Misses            ?       42           
  Partials          ?        0

Impacted Files	Coverage Δ
pennylane/beta/plugins/__init__.py	100% <100%> (ø)
pennylane/beta/plugins/default_tensor.py	97.68% <66.66%> (ø)
pennylane/beta/plugins/default_tensor_tf.py	92.03% <71.42%> (ø)

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[antalszava]

Looks good to me! Found one other occurrence, other than that it is a thorough renaming :) Would this device still be in an experimental phase, though it's name is changing?

[josh146]

Yep, I would still consider it experimental as it does not support all the features required of a PennyLane device (i.e. sampling and non analytic mode)