Add LongCat-Flash (#40730)
* working draft for LongCat
* BC changes to deepseek_v3 for modular
* format
* various modularities
* better tp plan
* better init
* minor changes
* make modular better
* clean up patterns
* Revert a couple of modular commits, because we won't convert in the end
* make things explicit.
* draft test
* toctree, tests and imports
* drop
* woops
* make better things
* update test
* update
* fixes
* style and CI
* convert stuff
* up
* ah, yes, that
* enable gen tests
* fix cache shape in test (sum of 2 things)
* fix tests
* comments
* re-Identitise
* minimize changes
* better defaults
* modular betterment
* fix configuration, add documentation
* fix init
* add integration tests
* add info
* simplify
* update slow tests
* fix
* style
* some additional long tests
* cpu-only long test
* fix last tests?
* urg
* cleaner tests why not
* fix
* improve slow tests, no skip
* style
* don't upcast
* one skip
* finally fix parallelism
