Was Stalin partly responsible for the bombing Hiroshima?
No, or perhaps I should say only by the furthest stretch of inference. I suppose it's possible to say that had the Russians become more involved in the war against Japan earlier perhaps the atomic bomb would not have been needed, but that really is stretching credulity as far as possible. The truth is that Stalin was, of course, completely unaware of the progress of the American atom project, which has become known as the "Manhattan Project," although "Manhattan City Project" was only one of many monthly-changed code words for the project. The Russians simply had their hands full with the Germans, plus little side projects like the 1943 deportation of the entire population of Chechnya to Siberia, and other Soviet ethnic cleansing programs like that.
In many respects the Soviet government and Stalin personally felt that the Western Allies had left them to fight the Germans "alone" for too long, and so left the Japanese more or less to the US. And of course the beating they had received in the 1904 Russo-Japanese War was also on their minds. The Soviets did invade the Kurile Islands and seized them from Japan, but rather belatedly.
The real reason for the atomic bombing of Hiroshima and Nagasaki was simply that the US wanted to make it as clear as possible to the Japanese military that it would be in the interest of their people if the war were brought to an immediate end. The Japanese military had control of the government and intended to resist the invasion of their homeland by total resistence with all available force, including the entire civil population. They encouraged the slogan "Ten Million Die Together." American intelligence suggested probably up to a million casualties in the invasion. Even that would not have guaranteed an end to the war, as the Japanese Army had over one million armed soldiers on the mainland in China, Korea and the French colonies in Indochina at the time. Although the Americans really only had the two bombs (one uranium and one plutonium), they used both and pretended to have more to force the decision. The evening after the bombing of Nagasaki, in a cabinet meeting, the Emperor spoke directly to the ministers and said,
"I have given serious thought to the situation prevailing at home and abroad and have concluded that continuing the war means destruction for the nation and a prolongation of bloodshed and cruelty for the world. I cannot bear to see my innocent people suffer any longer. Ending the war is the only way to restore world peace and to relieve the nation from the terrible distress with which it is burdened."
Although there was a palace revolt by hardliners, within days the Emperor Hirohito, supported by Keeper of the Privy Seal Koisi Kido, forced through his view. Interestingly, after the war Admiral Toyoda claimed that the invasion by Russian troops is what convinced Japan that the Allies' terms must be accepted sooner or later. The War Against Japan, the British official history of the war, also affirms this view, although it's doubtful the public knew of the invasion. I would advise anyone interested in understanding the Japanese side of the war to read The Rising Sun, by John Toland, certainly the finest English language history of the Japanese in the Pacific war.