[Kernel] Fix bug in Einsum implementation when a lone operand had a reduction operation #8486

Job	Run time
5. Build Extended Minimal	1m 21s
4. Build Minimal (Globally Allowed Types)	4m 19s
6c. Build Extended Minimal (No Optional Features)	1m 9s
2. Build Minimal (Exceptions Disabled)	1s
1. Build Full ORT and Generate ORT Files	4m 37s
6a. Build Regular (No Optional Features)	4m 37s
6b. Build Minimal (No Optional Features)	1s
3b. Build Minimal (Type Reduction)	1s
3a. Build Minimal (Custom Ops)	1s
7. Build Extended Minimal (Android NNAPI)	1s
	16m 8s

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