chore: move `LearningRateExp` to `deepmd.utils.learning_rate` #6891

Job	Run time
Build C library (2.15, libdeepmd_c.tar.gz)	7m 3s
Build C library (2.14, >=2.5.0rc0,<2.15, libdeepmd_c_cu11.tar.gz)	7m 19s
Test building from C library	1m 5s
Pass building c library	2s
	15m 29s

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