Add libsvm format design doc (#2554)
Add libsvm format design doc (#2554)

* add libsvm format design doc

* update

* change Tensor -> tensor

* update

* update to follow comments

* update

* update to remove LibSVM

* update

* change tensor -> vector
sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow