THOUGH he may seem like the Golden Child, Tom Hanks is in there wrestling with contrary fortune like any other actor. Only Hanks usually manages a pin before the bell rings. With his dead-on performances in last year's Sleepless in Seattle and this year's Philadelphia , Hanks finds himself in a very select class. Only Michael Keaton seems to be giving Hanks any competition as a nifty actor who can do comedy and drama with equal expertise. In fact, Keaton turned down the role of the nerdy suitor in Splash , the part that really boosted Hanks above his Bosom Buddies reputation. But for Hanks, it is all the same game. ''They grade on a bell curve here,'' says Hanks. ''I think the odds of having a success beyond your wildest dreams are the same as having a failure beyond your wildest nightmare. ''The risks are constant. They never go away. They never ease up. You never get a free ride. You always have to prove yourself. ''At the same time you can't pay too much attention to what you just did - either the success or failure of it - because it's a marathon. You put your head down and every time you look up you still have about 22 miles to go before you're all done.''