Fix as_tensor in onnx_text_plot_tree

[xadupre]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.78%. Comparing base (96eb50e) to head (fd5fc82).

Files with missing lines	Patch %	Lines
onnx_array_api/plotting/text_plot.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
- Coverage   81.89%   81.78%   -0.12%     
==========================================
  Files          63       63              
  Lines        8370     8373       +3     
==========================================
- Hits         6855     6848       -7     
- Misses       1515     1525      +10     

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