Remove size parameter for GATv2 and HEAT

[saiden89]

As proposed in #3713, this PR removes the size argument from the forward methods of the two classes and edits the tests to pass accordingly.

p.s. PR #3743 was forked from the wrong branch. Sorry for the inconvenience :(

[codecov-commenter]

Codecov Report

Merging #3744 (86c16f6) into master (95ef04f) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3744      +/-   ##
==========================================
- Coverage   81.79%   81.79%   -0.01%     
==========================================
  Files         299      299              
  Lines       15099    15097       -2     
==========================================
- Hits        12350    12348       -2     
  Misses       2749     2749              

Impacted Files	Coverage Δ
torch_geometric/nn/conv/gatv2_conv.py	91.81% <100.00%> (-0.15%)	⬇️
torch_geometric/nn/conv/heat_conv.py	100.00% <100.00%> (ø)

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[rusty1s]

Thank you!