Correct the use of DeviceContext in unittest sequence_pooling_test and sequence_padding_test (#22456)

* Add log in memory::Copy for debug purpose.

* Change to use context in DeviceContextPool directly in sequence_pooling_test, instead to new one.

* Change to use context in DeviceContextPool directly in sequence_padding_test, instead to new one.
test=develop

* Change the type of second_dim from size_t to int64_t.
test=develop