[Executorch][portable] Split op_mul in impl plus op

[kimishpatel]

Stack from ghstack (oldest at bottom):

• [Executorch] Renable operator optimization flags #6715
• [Executorch][Optimized] Use portable's impl for optimized op_add's fallback #6737
• [Exeuctorch][Portale] Refactor op_sub to be reused by optimized fallback #6736
• [Executorch][Optimized] Use portable's impl for optimized op_add's fallback #6735
• [Exeuctorch][Portale] Refactor op_add to be reused by optimized fallback #6734
• [Exeuctorch][Optimized] Use portable path for fallback in op_mul #6733
• -> [Executorch][portable] Split op_mul in impl plus op #6732
• [Executorch][optimized] Fix op_div impl to use portable for fallback path #6714
• Refactor op_div for shreability with optimized op_div #6713

This is to facilitate reuse of portable for optimized mul's fallback

Differential Revision: D65628859

[pytorch-bot]

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