As long as these are mere fleeting thoughts and you do not bring them upon the tongue you cannot be considered to have committed Shirk. The Messenger of Allah (peace and blessings be upon him) said: “Verily Allah (the Great and Mighty) has pardoned for my Ummah the thoughts that come to them, so long as they do not act upon them or utter them” (Sahih Muslim).

In fact, these Waswasahs/evil thoughts, like other vile thoughts, are from Shaytan and are actually a good sign that confirms to a person that he is upon Imaan, as these thoughts do not come to the disbeliever, only the believer. In a hadith also narrated in Sahih al-Muslim, a group of companions came to the Messenger of Allah (peace and blessings be upon him) complaining: “We think sometimes of a thing which each one of us would consider heinous to utter.” The Messenger of Allah (peace and blessings be upon him) replied: “That is clear Iman.” The Messenger of Allah (peace and blessings be upon) thus reassured them that you fear that these thoughts somehow negate your Imaan, but the fact that you at the same time, when you have them, feel they are heinous is the best evidence that you have Imaan, (Takmilah Fath al-Mulhim, vol.2 p.209).
Shirk means to ascribe partners to Allah taala, whether this be in His Sifat (Attributes) or in his being the only Ilaah (God) also known as Shirk fil-Uloohiyyah. Thus an example of the first category, is to consider a human to have perfect knowledge of the Unseen or to consider a person to be present everywhere and anywhere he wishes by His own wish or to be able to independently fulfil the need of others. An example of the second category would be the case of Christians believing Isa (a.s.) to be a God. Committing shirk of either category will take a person out of Islam.