Give DLDeviceType a sentinel value

[cconvey]

The DLDeviceType enum enumerates the list of device types.

The TVM project defines another enumeration, TVMDeviceExtType, that provides a supplemental set of devices / enumerators. It's important that there's no overlap of the integers provided by DLDeviceType and TVMDeviceExtType.

Unfortunately there's currently no good mechanism to notice when changes to either project lead to both using the same integer value in those enumerations.

We could address this by adding a sentinel value to DLDeviceType, e.g.:

typedef enum {
  kDLDeviceType_Begin = 1,
  kDLCPU = kDLDeviceType_Begin,
   ...
  kDLWebGPU = 15,
  /*! \brief Qualcomm Hexagon DSP */
  kDLHexagon = 16,
  kDLDeviceType_End, // all DLDeviceType enumerators are guaranteed to be numerically lower than this integer
} DLDeviceType;   

With this in place, TVM could safely avoid problems using something like this:

typedef enum {
  kDLAOCL = kDLDeviceType_End,
  kDLSDAccel,
  kOpenGL,
  kDLMicroDev,
  kDLWebGPU,
  // AddExtraTVMType which is not in DLPack here
} TVMDeviceExtType;

or this:

typedef enum {
  kDLAOCL = ...,
  ...
} TVMDeviceExtType;

// Relies on kDLAOCL having the lowest integer value in TVMDeviceExtType.
static_assert(KDLAOCL > kDLEnumEnd);
``

[cconvey]

Update: I noticed that TVM also assumes that:

• DLDeviceType does not define enumerators for -1 and 0, but
• the compiler does pick an underlying data type that's capable of expressing -1 and 0.

Probably the right solution is for TVM to stop making such strong assumptions about this enumeration, especially in code that needs to compile as C rather than C++.

But if DLPack wants to help TVM avoid problems, then this enumeration should provide both kDLDeviceType_Begin and kDLDeviceType_End.