[Kernel] Fix `GroupNormGradKernel` when `d_x` is nullptr #4751

Job	Run time
Clone-linux / Clone Paddle	5m 6s
Coverage / Coverage build	42m 30s
PR-CI-SOT / Check bypass for SOT / Check bypass	3s
PR-CI-Inference / Check bypass for inference / Check bypass	6s
Linux-XPU / Check bypass for XPU / Check bypass	3s
Linux-CPU / Build and test	31m 1s
PR-CI-SOT / Build and Test	57m 13s
Linux-XPU / Build	56m 6s
PR-CI-Inference / Build	17m 46s
PR-CI-Inference / Test	7m 44s
Linux-NPU / Check bypass for NPU / Check bypass	4s
Linux-NPU / Test	7m 34s
Coverage / Coverage test	1h 50m 29s
Linux-XPU / Test	37m 5s
	6h 12m 50s

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