reset the graph before freezing the compressed model

[njzjz]

A graph has a size limitation of 2 GB, so if we do not clean the graph, the limitation will be indeed 1GB...

This PR will increase the limitation from 1 GB to 2 GB.

[codecov-commenter]

Codecov Report

Merging #1658 (57dbfca) into devel (de7ba72) will decrease coverage by 11.86%.
The diff coverage is n/a.

❗ Current head 57dbfca differs from pull request most recent head 8e1895d. Consider uploading reports for the commit 8e1895d to get more accurate results

@@             Coverage Diff             @@
##            devel    #1658       +/-   ##
===========================================
- Coverage   76.15%   64.28%   -11.87%     
===========================================
  Files          94        5       -89     
  Lines        7850       14     -7836     
===========================================
- Hits         5978        9     -5969     
+ Misses       1872        5     -1867     

Impacted Files	Coverage Δ
deepmd/entrypoints/compress.py
deepmd/loss/ener.py
deepmd/utils/data.py
deepmd/cluster/local.py
deepmd/utils/path.py
deepmd/infer/deep_polar.py
source/op/_prod_virial_se_a_grad.py
deepmd/env.py
deepmd/entrypoints/train.py
source/op/_prod_force_grad.py
... and 79 more

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