See Could a planet form without a molten state?, where Anders Sandberg calculates that a body with radius ~ 1000 km would melt from the heat of formation.
The largest asteroid, Ceres, has a radius of 473 km, so it's a bit too small to have totally melted just from gravitational accretion, but it may have picked up a little more energy from impacts that were faster than freefall. It could also generate some internal heat from radioactive decay, but it's not expected to contain a significant percentage of radioactive material.
Presumably, Ceres was plastic to some degree during formation, since it's (almost) spherical (and it's the only asteroid large enough for gravity to make it spherical). But that wouldn't have required a very high temperature, since Ceres contains a lot of water ice.
But even if it did totally melt when it formed, it's had over 4 billion years to cool down, so it's extremely unlikely to still have a hot core. However, it does emit more water vapour than expected, and it may be warm enough to contain some liquid water below the surface.