guard dimname after removal (#20071) #249

Job	Run time
test-webgpu / test-backend-linux (webgpu, models) / linux-job	16m 51s
test-webgpu / test-backend-linux (webgpu, operators) / linux-job	37m 21s
test-webgpu / test-backend-macos	-1s
test-webgpu / package-golden-artifacts	1m 39s
	55m 50s

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