[FSDP2] Used `CommDebugMode` for comm. count test

[awgu]

Stack from ghstack (oldest at bottom):

• #126892
• -> #126887

simplify the test :)

cc @mrshenli @pritamdamania87 @zhaojuanmao @satgera @gqchen @aazzolini @osalpekar @jiayisuse @H-Huang @kwen2501 @penguinwu @fegin @XilunWu @wanchaol @fduwjj @wz337 @tianyu-l @wconstab @yf225 @chauhang @d4l3k

[pytorch-bot[bot]]

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As of commit e20c6d3a56665980d047beaf60ab02b615932cb9 with merge base 4575d3be832d8bbb5ae031155ca23dbf584ec1be (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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[awgu]

@pytorchbot merge