[MoE Layer] Support MoE Drop Tokens

[hushenwei2000]

支持丢 Tokens 逻辑，与 PaddleFormers PaddlePaddle/PaddleFormers#3291 对齐。

• 新增 moe_token_drop_policy
• 新增 moe_expert_capacity_factor

[codecov-commenter]

Codecov Report

❌ Patch coverage is 28.57143% with 20 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (develop@938973c). Learn more about missing BASE report.

⚠️ Current head 091a08d differs from pull request most recent head eb0d061

Please upload reports for the commit eb0d061 to get more accurate results.

Files with missing lines	Patch %	Lines
src/paddlefleet/transformer/moe/moe_router.py	28.57%	19 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (28.57%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #248   +/-   ##
==========================================
  Coverage           ?   28.57%           
==========================================
  Files              ?        1           
  Lines              ?       28           
  Branches           ?        5           
==========================================
  Hits               ?        8           
  Misses             ?       19           
  Partials           ?        1           

Flag	Coverage Δ
coverage_combine	28.57% <28.57%> (?)

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Files with missing lines	Coverage Δ
src/paddlefleet/transformer/moe/moe_router.py	28.57% <28.57%> (ø)

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