guard dimname after removal (#20071) #255

Job	Run time
test-webgpu / test-backend-linux (webgpu, models) / linux-job	17m 9s
test-webgpu / test-backend-linux (webgpu, operators) / linux-job	37m 30s
test-webgpu / test-backend-macos	0s
test-webgpu / package-golden-artifacts	1m 29s
	56m 8s

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