Support fill_value for SimpleImputer with string data #109

Job	Run time
windows-latest py==3.11 - sklearn==1.5.0 - onnxruntime<1.19.0	7m 34s
windows-latest py==3.11 - sklearn==1.4.2 - onnxruntime>=1.18.0	7m 20s
windows-latest py==3.11 - sklearn==1.3.2 - onnxruntime<1.18.0	6m 49s
windows-latest py==3.9 - sklearn==1.2.2 - onnxruntime<1.16.0	7m 32s
windows-latest py==3.11 - sklearn==1.1.3 - onnxruntime>=1.18.0	6m 6s
macos-latest py==3.11 - sklearn==1.5.0 - onnxruntime<1.19.0	2m 28s
macos-latest py==3.11 - sklearn==1.4.2 - onnxruntime>=1.18.0	3m 16s
macos-latest py==3.11 - sklearn==1.3.2 - onnxruntime<1.18.0	2m 33s
macos-latest py==3.9 - sklearn==1.2.2 - onnxruntime<1.16.0	3m 40s
macos-latest py==3.11 - sklearn==1.1.3 - onnxruntime>=1.18.0	2m 22s
	49m 40s

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