Plan TP=1 with EP>1 simulation fix

[Copilot]

Simulations with TP=1 and EP>1 lose EP/DP_EP group membership and suppress forward/input-gradient EP collectives.

• Root cause
  • SimCCL derives EP/DP_EP groups from TP groups, but omits singleton TP groups.
  • SimAI applies the empty TP dimension mask to EP collectives.
• Proposed changes
  • Represent TP=1 ranks as singleton TP groups in SimCCL.
  • Use data-parallel dimensions for EP forward/input-gradient collectives.
  • Preserve existing TP=1 communicator suppression and TP>1 behavior.
• Scope
  • Analysis and implementation plan only; this PR currently contains no code changes.

• Fixes TP=1,EP>1时仿真失败 #298

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