Helen Hunt has been acting since age 8, won an Academy Award for 1997’s “As Good as It Gets,” and scored four Emmy Awards for her work on “Mad About You.” So it is no small compliment to say that her performance in “The Sessions” ranks among the best work of her career. In the fact-based film, Hunt plays sex surrogate Cheryl Cohen-Greene, who is called upon by polio survivor Mark O’Brien (John Hawkes). What begins as a clinical relationship soon goes much deeper, as Hunt’s nakedness transforms from physical to emotional.