How many different directors are associated with the writer Gaby Chiappe?
CREATE TABLE "table1_15026994_2" (
  "num" real,
  "episode" text,
  "writer" text,
  "director" text,
  "original_air_date" text,
  "viewing_figure" text
);
SELECT COUNT("director") FROM "table1_15026994_2" WHERE "writer"='Gaby Chiappe';