Add ROCm support (AMDGPU)

src/rocm.jl

@@ -1,11 +1,11 @@
 import .AMDGPU
 
 function Base.cconvert(::Type{MPIPtr}, A::AMDGPU.ROCArray{T}) where T
-    Base.cconvert(Ptr{T}, A.buf.ptr) # returns DeviceBuffer
+    A
 end
 
 function Base.unsafe_convert(::Type{MPIPtr}, X::AMDGPU.ROCArray{T}) where T
-    reinterpret(MPIPtr, Base.unsafe_convert(Ptr{T}, X.buf.ptr))
+    reinterpret(MPIPtr, Base.unsafe_convert(Ptr{T}, X.buf.ptr+X.offset))
 end
 
 # only need to define this for strided arrays: all others can be handled by generic machinery