The Biden administration is considering reviving the practice of detaining migrant families who cross the border illegally — the same policy the president shut down over the past two years because he wanted a more humane immigration system, officials familiar with the discussions said Monday. Although no final decision has been made, the move would be a stark reversal for President Biden, who came into office promising to adopt a more compassionate approach to the border after the harsh policies of his predecessor, former President Donald J. Trump.